I finally built my set-up with the LDR (were lying in my working room for months...). Conclusion: it works!

I will give it a try. I ordered also the LDR variants of these.
M.

I use this one:

I used earlier which I made myself, but on a raspberry pi. Reading out analogue, passed it through a ADC and fed it to the pi.

Now use the one above and linked it to the mega. Adjusting the resistance by the pot-meter will get me always on or always off.

I managed to get it working. First issue was to get the Mega working with the mega.

I needed to change MyConfig.h:

#define MY_RF24_CS_PIN 53

I used CE on pin 9 (and not 40) and further more:

My sketch:

// Enable debug prints
#define MY_DEBUG

// Enable and select radio type attached
#define MY_RADIO_NRF24

#include <MySensors.h>  
// electricity
#define DIGITAL_INPUT_SENSOR_e 18  // The digital input you attached your light sensor.  (Only 2 and 3 generates interrupt!)
#define PULSE_FACTOR_e 1000        // Nummber of blinks per KWH of your meeter
#define SLEEP_MODE false           // Watt-value can only be reported when sleep mode is false.
#define MAX_WATT_e 10000           // Max watt value to report. This filetrs outliers.
#define CHILD_ID_e 1               // Id of the sensor child

// Water
#define DIGITAL_INPUT_SENSOR_w 19                 // The digital input you attached your sensor.  (Only 2 and 3 generates interrupt!)
#define PULSE_FACTOR_w 1000                       // Nummber of blinks per m3 of your meter (One rotation/liter)
#define MAX_FLOW_w 40                             // Max flow (l/min) value to report. This filters outliers.
#define CHILD_ID_w 2                              // Id of the sensor child


// Gas
#define DIGITAL_INPUT_SENSOR_g 20                 // The digital input you attached your sensor.  (Only 2 and 3 generates interrupt!)
#define PULSE_FACTOR_g 1000                       // Nummber of blinks per m3 of your meter (One rotation/liter)
#define MAX_FLOW_g 40                             // Max flow (l/min) value to report. This filters outliers.
#define CHILD_ID_g 3                              // Id of the sensor child


unsigned long SEND_FREQUENCY = 30000;           // Minimum time between send (in milliseconds). We don't want to spam the gateway.
//unsigned long SEND_FREQUENCY = 20000; // Minimum time between send (in milliseconds). We don't wnat to spam the gateway.


double ppwh = ((double)PULSE_FACTOR_e)/1000; // Pulses per watt hour
double pplw = ((double)PULSE_FACTOR_w)/1000; 
double pplg = ((double)PULSE_FACTOR_g)/1000; 

//electricity
bool pcReceived_e = false;
volatile unsigned long pulseCount_e = 0;   
volatile unsigned long lastBlink_e = 0;
volatile unsigned long watt = 0;
unsigned long oldPulseCount_e = 0;   
unsigned long oldWatt = 0;
double oldKwh;
unsigned long lastSend_e;


// water
volatile unsigned long pulseCount_w = 0;   
volatile unsigned long lastBlink_w = 0;
volatile double flow_w = 0;  
bool pcReceived_w = false;
unsigned long oldPulseCount_w = 0;
unsigned long newBlink_w = 0;   
double oldflow_w = 0;
double volume_w =0;                     
double oldvolume_w =0;
unsigned long lastSend_w =0;
unsigned long lastPulse_w =0;

// gas
volatile unsigned long pulseCount_g = 0;   
volatile unsigned long lastBlink_g = 0;
volatile double flow_g = 0;  
bool pcReceived_g = false;
unsigned long oldPulseCount_g = 0;
unsigned long newBlink_g = 0;   
double oldflow_g = 0;
double volume_g =0;                     
double oldvolume_g =0;
unsigned long lastSend_g =0;
unsigned long lastPulse_g =0;


// electricity
MyMessage wattMsg(CHILD_ID_e,V_WATT);
MyMessage kwhMsg(CHILD_ID_e,V_KWH);
MyMessage pcMsg(CHILD_ID_e,V_VAR1);
// Water
MyMessage waterflowMsg(CHILD_ID_w,V_FLOW);
MyMessage watervolumeMsg(CHILD_ID_w,V_VOLUME);
MyMessage waterlastCounterMsg(CHILD_ID_w,V_VAR2);
// Gas
MyMessage gasflowMsg(CHILD_ID_g,V_FLOW);
MyMessage gasvolumeMsg(CHILD_ID_g,V_VOLUME);
MyMessage gaslastCounterMsg(CHILD_ID_g,V_VAR3);



void setup()  
{  
  // Fetch last known pulse count value from gw
  request(CHILD_ID_e, V_VAR1);
  request(CHILD_ID_w, V_VAR2);
  request(CHILD_ID_g, V_VAR3);

  // Use the internal pullup to be able to hook up this sketch directly to an energy meter with S0 output
  // If no pullup is used, the reported usage will be too high because of the floating pin
  pinMode(DIGITAL_INPUT_SENSOR_e,INPUT_PULLUP);
  pinMode(DIGITAL_INPUT_SENSOR_w,INPUT_PULLUP);
  pinMode(DIGITAL_INPUT_SENSOR_g,INPUT_PULLUP);

  attachInterrupt(digitalPinToInterrupt(DIGITAL_INPUT_SENSOR_e), onPulse_e, RISING);
  attachInterrupt(digitalPinToInterrupt(DIGITAL_INPUT_SENSOR_w), onPulse_w, RISING);
  attachInterrupt(digitalPinToInterrupt(DIGITAL_INPUT_SENSOR_g), onPulse_g, RISING);
  lastSend_e=millis();
  lastSend_w=millis();
  lastSend_g=millis();
}

void presentation() {
  // Send the sketch version information to the gateway and Controller
  sendSketchInfo("Meterkast Marcel", "1.0");

  // Register this device as power sensor
  present(CHILD_ID_e, S_POWER);
  present(CHILD_ID_w, S_WATER);
  present(CHILD_ID_g, S_GAS);
}

void loop()     
{ 
  unsigned long now = millis();
  // Only send values at a maximum frequency or woken up from sleep
  bool sendTime = now - lastSend_e > SEND_FREQUENCY;
  if (pcReceived_e && (SLEEP_MODE || sendTime)) {
    // New watt value has been calculated  
    if (!SLEEP_MODE && watt != oldWatt) {
      // Check that we dont get unresonable large watt value. 
      // could hapen when long wraps or false interrupt triggered
      if (watt<((unsigned long)MAX_WATT_e)) {
        send(wattMsg.set(watt));  // Send watt value to gw 
      }  
      Serial.print("Watt:");
      Serial.println(watt);
      oldWatt = watt;
    }

    // Pulse count has changed
    if (pulseCount_e != oldPulseCount_e) {
      send(pcMsg.set(pulseCount_e));  // Send pulse count value to gw 
      double kwh = ((double)pulseCount_e/((double)PULSE_FACTOR_e));     
      oldPulseCount_e = pulseCount_e;
      if (kwh != oldKwh) {
        send(kwhMsg.set(kwh, 4));  // Send kwh value to gw 
        oldKwh = kwh;
      }
    }    
    lastSend_e = now;
  } else if (sendTime && !pcReceived_e) {
    // No count received. Try requesting it again
    request(CHILD_ID_e, V_VAR1);
    lastSend_e=now;
  }

    unsigned long currentTime = millis();

    // Only send values at a maximum frequency or woken up from sleep
  if (SLEEP_MODE || (currentTime - lastSend_w > SEND_FREQUENCY))
  {
    lastSend_w=currentTime;

    if (!pcReceived_w) {
      //Last Pulsecount not yet received from controller, request it again
      request(CHILD_ID_w, V_VAR2);
      return;
    }

    if (!SLEEP_MODE && flow_w != oldflow_w) {
      oldflow_w = flow_w;

      Serial.print("l/min:");
      Serial.println(flow_w);

      // Check that we dont get unresonable large flow value. 
      // could hapen when long wraps or false interrupt triggered
      if (flow_w<((unsigned long)MAX_FLOW_w)) {
        send(waterflowMsg.set(flow_w, 2));                   // Send flow value to gw
      }  
    }

    // No Pulse count received in 2min 
    if(currentTime - lastPulse_w > 120000){
      flow_w = 0;
    } 

    // Pulse count has changed
    if ((pulseCount_w != oldPulseCount_w)||(!SLEEP_MODE)) {
      oldPulseCount_w = pulseCount_w;

      Serial.print("pulsecount_w:");
      Serial.println(pulseCount_w);

      send(waterlastCounterMsg.set(pulseCount_w));                  // Send  pulsecount value to gw in VAR1

      double volume_w = ((double)pulseCount_w/((double)PULSE_FACTOR_w));     
      if ((volume_w != oldvolume_w)||(!SLEEP_MODE)) {
        oldvolume_w = volume_w;

        Serial.print("volume_w:");
        Serial.println(volume_w, 3);

        send(watervolumeMsg.set(volume_w, 3));               // Send volume value to gw
      } 
    }
  }

    // Only send values at a maximum frequency or woken up from sleep
  if (SLEEP_MODE || (currentTime - lastSend_g > SEND_FREQUENCY))
  {
    lastSend_g=currentTime;

    if (!pcReceived_g) {
      //Last Pulsecount not yet received from controller, request it again
      request(CHILD_ID_g, V_VAR3);
      return;
    }

    if (!SLEEP_MODE && flow_g != oldflow_g) {
      oldflow_g = flow_g;

      Serial.print("l/min:");
      Serial.println(flow_g);

      // Check that we dont get unresonable large flow value. 
      // could hapen when long wraps or false interrupt triggered
      if (flow_g<((unsigned long)MAX_FLOW_g)) {
        send(gasflowMsg.set(flow_g, 2));                   // Send flow value to gw
      }  
    }

    // No Pulse count received in 2min 
    if(currentTime - lastPulse_g > 120000){
      flow_g = 0;
    } 

    // Pulse count has changed
    if ((pulseCount_g != oldPulseCount_g)||(!SLEEP_MODE)) {
      oldPulseCount_g = pulseCount_g;

      Serial.print("pulsecount_g:");
      Serial.println(pulseCount_g);

      send(gaslastCounterMsg.set(pulseCount_g));                  // Send  pulsecount value to gw in VAR1

      double volume_g = ((double)pulseCount_g/((double)PULSE_FACTOR_g));     
      if ((volume_g != oldvolume_g)||(!SLEEP_MODE)) {
        oldvolume_g = volume_g;

        Serial.print("volume_g:");
        Serial.println(volume_g, 3);

        send(gasvolumeMsg.set(volume_g, 3));               // Send volume value to gw
      } 
    }
  }
  
  
  if (SLEEP_MODE) {
    sleep(SEND_FREQUENCY);
  }
}

void receive(const MyMessage &message) {
  if (message.type==V_VAR1) {  
    pulseCount_e = oldPulseCount_e = message.getLong();
    Serial.print("Received last pulse count from gw_e:");
    Serial.println(pulseCount_e);
    pcReceived_e = true;
  }
  if (message.type==V_VAR2) {
    unsigned long gwPulseCount_w=message.getULong();
    pulseCount_w += gwPulseCount_w;
    flow_w=oldflow_w=0;
    Serial.print("Received last pulse count from gw_w:");
    Serial.println(pulseCount_w);
    pcReceived_w = true;
  }
  if (message.type==V_VAR3) {
    unsigned long gwPulseCount_g=message.getULong();
    pulseCount_g += gwPulseCount_g;
    flow_g=oldflow_g=0;
    Serial.print("Received last pulse count from gw_g:");
    Serial.println(pulseCount_g);
    pcReceived_g = true;
  }
  
}

void onPulse_e()     
{ 
  if (!SLEEP_MODE) {
    unsigned long newBlink_e = micros();  
    unsigned long interval_e = newBlink_e-lastBlink_e;
    if (interval_e<10000L) { // Sometimes we get interrupt on RISING
      return;
    }
    watt = (3600000000.0 /interval_e) / ppwh;
    lastBlink_e = newBlink_e;
  } 
  pulseCount_e++;
}

void onPulse_w()     
{
  if (!SLEEP_MODE)
  {
    unsigned long newBlink_w = micros();   
    unsigned long interval_w = newBlink_w-lastBlink_w;

    if (interval_w!=0)
    {
      lastPulse_w = millis();
      if (interval_w<500000L) {
        // Sometimes we get interrupt on RISING,  500000 = 0.5sek debounce ( max 120 l/min)
        return;   
      }
      flow_w = (60000000.0 /interval_w) / pplw;
    }
    lastBlink_w = newBlink_w;
  }
  pulseCount_w++; 
}

void onPulse_g()     
{
  if (!SLEEP_MODE)
  {
    unsigned long newBlink_g = micros();   
    unsigned long interval_g = newBlink_g-lastBlink_g;

    if (interval_g!=0)
    {
      lastPulse_g = millis();
      if (interval_g<500000L) {
        // Sometimes we get interrupt on RISING,  500000 = 0.5sek debounce ( max 120 l/min)
        return;   
      }
      flow_g = (60000000.0 /interval_g) / pplg;
    }
    lastBlink_g = newBlink_g;
  }
  pulseCount_g++; 
}

I still got one issue; my led on the meter does not give enough light to have a n interrupt. does anybody have a suggestion how to get the interrupt?

Thanks! I ordered 2 mega's to experiment....

@ericvdb What's the difference between a interrupt and a digital input (I am a civil engineer with some programming background)

@ericvdb So the only solution is to use 2 nano's?

@ericvdb but it should be possible to make a 3rd one available I think?

I managed to have the script compiled without errors. But I found out that the 3rd sensor is not checked or giving interrupts (the led on the sensor indicates that there are pulses). With playing around and checking some regular sketches, I made a mistake on my breadboard and I burnt my nano :-(. So I ordered a new one.....
Script so far is shown below, but it might be that I have to built a script from scratch with mysensors api. If anyone has any directions to follow on this, it would be great!

/**
 * The MySensors Arduino library handles the wireless radio link and protocol
 * between your home built sensors/actuators and HA controller of choice.
 * The sensors forms a self healing radio network with optional repeaters. Each
 * repeater and gateway builds a routing tables in EEPROM which keeps track of the
 * network topology allowing messages to be routed to nodes.
 *
 * Created by Henrik Ekblad <henrik.ekblad@mysensors.org>
 * Copyright (C) 2013-2015 Sensnology AB
 * Full contributor list: https://github.com/mysensors/Arduino/graphs/contributors
 *
 * Documentation: http://www.mysensors.org
 * Support Forum: http://forum.mysensors.org
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * version 2 as published by the Free Software Foundation.
 *
 *******************************
 *
 * REVISION HISTORY
 * Version 1.0 - Henrik EKblad
 * 
 * DESCRIPTION
 * This sketch provides an example how to implement a distance sensor using HC-SR04 
 * Use this sensor to measure KWH and Watt of your house meeter
 * You need to set the correct pulsefactor of your meeter (blinks per KWH).
 * The sensor starts by fetching current KWH value from gateway.
 * Reports both KWH and Watt back to gateway.
 *
 * Unfortunately millis() won't increment when the Arduino is in 
 * sleepmode. So we cannot make this sensor sleep if we also want 
 * to calculate/report watt-number.
 * http://www.mysensors.org/build/pulse_power
 */

// Enable debug prints
#define MY_DEBUG

// Enable and select radio type attached
#define MY_RADIO_NRF24
//#define MY_RADIO_RFM69

#include <MySensors.h>  
#define SLEEP_MODE false        // values can only be reported when sleep mode is false.

//electriciteit
#define DIGITAL_INPUT_SENSOR_e 2  // The digital input you attached your light sensor.  (Only 2 and 3 generates interrupt!)
#define PULSE_FACTOR_e 1000       // Nummber of blinks per KWH of your meeter
#define MAX_WATT 10000          // Max watt value to report. This filetrs outliers.
#define CHILD_ID_e 1              // Id of the sensor child

//water
#define DIGITAL_INPUT_SENSOR_w 3  // The digital input you attached your light sensor.  (Only 2 and 3 generates interrupt!)
#define PULSE_FACTOR_w 1000       // Nummber of blinks per KWH of your meeter
#define MAX_WATER 10000          // Max watt value to report. This filetrs outliers.
#define CHILD_ID_w 2              // Id of the sensor child

//gas
#define DIGITAL_INPUT_SENSOR_g 4  // The digital input you attached your light sensor.  (Only 2 and 3 generates interrupt!)
#define PULSE_FACTOR_g 1000       // Nummber of blinks per KWH of your meeter
#define MAX_GAS 10000          // Max watt value to report. This filetrs outliers.
#define CHILD_ID_g 3              // Id of the sensor child


unsigned long SEND_FREQUENCY = 20000; // Minimum time between send (in milliseconds). We don't wnat to spam the gateway.
double ppwh_e = ((double)PULSE_FACTOR_e)/1000; // Pulses per watt hour
double ppwh_w = ((double)PULSE_FACTOR_w)/1000; // Pulses per watt hour
double ppwh_g = ((double)PULSE_FACTOR_g)/1000; // Pulses per watt hour
bool pcReceived = false;
volatile unsigned long pulseCount_e = 0;   
volatile unsigned long pulseCount_w = 0;   
volatile unsigned long pulseCount_g = 0;   
volatile unsigned long lastBlink_e = 0;
volatile unsigned long lastBlink_w = 0;
volatile unsigned long lastBlink_g = 0;
volatile unsigned long watt = 0;
volatile unsigned long waterflow = 0;
volatile unsigned long gasflow = 0;
unsigned long oldPulseCount_e = 0;   
unsigned long oldPulseCount_w = 0;   
unsigned long oldPulseCount_g = 0;   
unsigned long oldWatt = 0;
unsigned long oldWaterf = 0;
unsigned long oldgasf = 0;
double oldKwh;
double oldwater;
double oldgas;
unsigned long lastSend;
MyMessage wattMsg(CHILD_ID_e,V_WATT);
MyMessage kwhMsg(CHILD_ID_e,V_KWH);
MyMessage pcMsg(CHILD_ID_e,V_VAR1);

MyMessage waterMsg(CHILD_ID_w,V_FLOW);
MyMessage watervolumeMsg(CHILD_ID_w,V_VOLUME);
MyMessage lastCounterwaterMsg(CHILD_ID_w,V_VAR2);

MyMessage gasMsg(CHILD_ID_g,V_FLOW);
MyMessage gasvolumeMsg(CHILD_ID_g,V_VOLUME);
MyMessage lastCountegasrMsg(CHILD_ID_g,V_VAR3);


void setup()  
{  
  // Fetch last known pulse count value from gw
  request(CHILD_ID_e, V_VAR1);
  request(CHILD_ID_w, V_VAR2);
  request(CHILD_ID_g, V_VAR3);

  // Use the internal pullup to be able to hook up this sketch directly to an energy meter with S0 output
  // If no pullup is used, the reported usage will be too high because of the floating pin
  pinMode(DIGITAL_INPUT_SENSOR_e,INPUT_PULLUP);
  pinMode(DIGITAL_INPUT_SENSOR_w,INPUT_PULLUP);
  pinMode(DIGITAL_INPUT_SENSOR_g,INPUT_PULLUP);

  attachInterrupt(digitalPinToInterrupt(DIGITAL_INPUT_SENSOR_e), onPulse_e, RISING);
  attachInterrupt(digitalPinToInterrupt(DIGITAL_INPUT_SENSOR_w), onPulse_w, RISING);
  attachInterrupt(digitalPinToInterrupt(DIGITAL_INPUT_SENSOR_g), onPulse_g, RISING);
  lastSend=millis();

}

void presentation() {
  // Send the sketch version information to the gateway and Controller
  sendSketchInfo("Marcel Meterkast", "1.0");

  // Register this device as power sensor
  present(CHILD_ID_e, S_POWER);
  present(CHILD_ID_w, S_WATER);
  present(CHILD_ID_g, S_GAS);

}

void loop()     
{ 
  
  unsigned long now = millis();
  
  // Only send values at a maximum frequency or woken up from sleep
  bool sendTime = now - lastSend > SEND_FREQUENCY;
  if (pcReceived && (SLEEP_MODE || sendTime)) {
    // New watt value has been calculated  
    // electricity first
    if (!SLEEP_MODE && watt != oldWatt) {
      // Check that we dont get unresonable large watt value. 
      // could hapen when long wraps or false interrupt triggered
      if (watt<((unsigned long)MAX_WATT)) {
        send(wattMsg.set(watt));  // Send watt value to gw 
      }  
      Serial.print("Watt:");
      Serial.println(watt);
      oldWatt = watt;
    }
    // send water
    if (!SLEEP_MODE && waterflow != oldWaterf) {
      // Check that we dont get unresonable large watt value. 
      // could hapen when long wraps or false interrupt triggered
      if (waterflow<((unsigned long)MAX_WATER)) {
        send(wattMsg.set(waterflow));  // Send watt value to gw 
      }  
      Serial.print("water:");
      Serial.println(waterflow);
      oldWaterf = waterflow;
    }

    // send gas
    if (!SLEEP_MODE && gasflow != oldgasf) {
      // Check that we dont get unresonable large watt value. 
      // could hapen when long wraps or false interrupt triggered
      if (gasflow<((unsigned long)MAX_GAS)) {
        send(wattMsg.set(gasflow));  // Send watt value to gw 
      }  
      Serial.print("gas:");
      Serial.println(gasflow);
      oldgas = gasflow;
    }
    
    // Pulse cout has changed
    if (pulseCount_e != oldPulseCount_e) {
      send(pcMsg.set(pulseCount_e));  // Send pulse count value to gw 
      double kwh = ((double)pulseCount_e/((double)PULSE_FACTOR_e));     
      oldPulseCount_e = pulseCount_e;
      if (kwh != oldKwh) {
        send(kwhMsg.set(kwh, 4));  // Send kwh value to gw 
        oldKwh = kwh;
      }
    }
    if (pulseCount_w != oldPulseCount_w) {
      send(pcMsg.set(pulseCount_w));  // Send pulse count value to gw 
      double water = ((double)pulseCount_w/((double)PULSE_FACTOR_w));     
      oldPulseCount_w = pulseCount_w;
      if (water != oldwater) {
        send(watervolumeMsg.set(water, 4));  // Send watervolume value to gw 
        oldwater = water;
      }
    }
    if (pulseCount_g != oldPulseCount_g) {
      send(pcMsg.set(pulseCount_g));  // Send pulse count value to gw 
      double gas = ((double)pulseCount_g/((double)PULSE_FACTOR_g));     
      oldPulseCount_g = pulseCount_g;
      if (gas != oldgas) {
        send(gasvolumeMsg.set(gas, 4));  // Send gas value to gw 
        oldgas = gas;
      }
    }    
    lastSend = now;
  } else if (sendTime && !pcReceived) {
    // No count received. Try requesting it again
    request(CHILD_ID_e, V_VAR1);
    request(CHILD_ID_w, V_VAR2);
    request(CHILD_ID_g, V_VAR3);
    lastSend=now;
  }

  if (SLEEP_MODE) {
    sleep(SEND_FREQUENCY);
  }
}

void receive(const MyMessage &message) {
  if (message.type==V_VAR1) {  
    pulseCount_e = oldPulseCount_e = message.getLong();
    Serial.print("Received last pulse count from gw for:");
    Serial.println(pulseCount_e);
    pcReceived = true;
  }
}

void onPulse_e()     
{ 
  Serial.println("electricity pulse!");
  if (!SLEEP_MODE) {
    unsigned long newBlink = micros();  
    unsigned long interval = newBlink-lastBlink_e;
    if (interval<10000L) { // Sometimes we get interrupt on RISING
      return;
    }
    watt = (3600000000.0 /interval) / ppwh_e;
    lastBlink_e = newBlink;
  } 
  pulseCount_e++;
}

void onPulse_w()     
{ 
  Serial.println("water pulse!");
  if (!SLEEP_MODE) {
    unsigned long newBlink = micros();  
    unsigned long interval = newBlink-lastBlink_w;
    if (interval<10000L) { // Sometimes we get interrupt on RISING
      return;
    }
    waterflow = (3600000000.0 /interval) / ppwh_w;
    lastBlink_w = newBlink;
  } 
  pulseCount_w++;
}

void onPulse_g()     
{ 
  Serial.println("gas pulse!");
  if (!SLEEP_MODE) {
    unsigned long newBlink = micros();  
    unsigned long interval = newBlink-lastBlink_g;
    if (interval<10000L) { // Sometimes we get interrupt on RISING
      return;
    }
    gasflow = (3600000000.0 /interval) / ppwh_g;
    lastBlink_g = newBlink;
  } 
  pulseCount_w++;
}```

I am trying to combine 3 pulse counters on 1 nano. I have a regular pulse counter based on a LDR, which counts the led pulses on the electricity meter and I have two pulse counters (one on the gas and one on the watermeter) counting the mirrors passages on the wheels.
I would like to combine those on one nano and I started with the example of the electricity meter and modified it for 3 sensors. Than I got stuck...

My errors starts here:

MyMessage flowMsg(CHILD_ID_g,V_FLOW);
MyMessage volumeMsg(CHILD_ID_g,V_VOLUME);
MyMessage lastCounterMsg(CHILD_ID_g,V_VAR3);

The full script:

/**
 * The MySensors Arduino library handles the wireless radio link and protocol
 * between your home built sensors/actuators and HA controller of choice.
 * The sensors forms a self healing radio network with optional repeaters. Each
 * repeater and gateway builds a routing tables in EEPROM which keeps track of the
 * network topology allowing messages to be routed to nodes.
 *
 * Created by Henrik Ekblad <henrik.ekblad@mysensors.org>
 * Copyright (C) 2013-2015 Sensnology AB
 * Full contributor list: https://github.com/mysensors/Arduino/graphs/contributors
 *
 * Documentation: http://www.mysensors.org
 * Support Forum: http://forum.mysensors.org
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * version 2 as published by the Free Software Foundation.
 *
 *******************************
 *
 * REVISION HISTORY
 * Version 1.0 - Henrik EKblad
 * 
 * DESCRIPTION
 * This sketch provides an example how to implement a distance sensor using HC-SR04 
 * Use this sensor to measure KWH and Watt of your house meeter
 * You need to set the correct pulsefactor of your meeter (blinks per KWH).
 * The sensor starts by fetching current KWH value from gateway.
 * Reports both KWH and Watt back to gateway.
 *
 * Unfortunately millis() won't increment when the Arduino is in 
 * sleepmode. So we cannot make this sensor sleep if we also want 
 * to calculate/report watt-number.
 * http://www.mysensors.org/build/pulse_power
 */

// Enable debug prints
#define MY_DEBUG

// Enable and select radio type attached
#define MY_RADIO_NRF24
//#define MY_RADIO_RFM69

#include <MySensors.h>  
#define SLEEP_MODE false        // values can only be reported when sleep mode is false.

//electriciteit
#define DIGITAL_INPUT_SENSOR_e 2  // The digital input you attached your light sensor.  (Only 2 and 3 generates interrupt!)
#define PULSE_FACTOR_e 1000       // Nummber of blinks per KWH of your meeter
#define MAX_WATT_e 10000          // Max watt value to report. This filetrs outliers.
#define CHILD_ID_e 1              // Id of the sensor child

//water
#define DIGITAL_INPUT_SENSOR_w 3  // The digital input you attached your light sensor.  (Only 2 and 3 generates interrupt!)
#define PULSE_FACTOR_w 1000       // Nummber of blinks per KWH of your meeter
#define MAX_WATER 10000          // Max watt value to report. This filetrs outliers.
#define CHILD_ID_w 2              // Id of the sensor child

//gas
#define DIGITAL_INPUT_SENSOR_g 4  // The digital input you attached your light sensor.  (Only 2 and 3 generates interrupt!)
#define PULSE_FACTOR_g 1000       // Nummber of blinks per KWH of your meeter
#define MAX_GAS 10000          // Max watt value to report. This filetrs outliers.
#define CHILD_ID_g 3              // Id of the sensor child


unsigned long SEND_FREQUENCY = 20000; // Minimum time between send (in milliseconds). We don't wnat to spam the gateway.
double ppwh_e = ((double)PULSE_FACTOR_e)/1000; // Pulses per watt hour
double ppwh_w = ((double)PULSE_FACTOR_w)/1000; // Pulses per watt hour
double ppwh_g = ((double)PULSE_FACTOR_g)/1000; // Pulses per watt hour
bool pcReceived = false;
volatile unsigned long pulseCount_e = 0;   
volatile unsigned long pulseCount_w = 0;   
volatile unsigned long pulseCount_g = 0;   
volatile unsigned long lastBlink_e = 0;
volatile unsigned long lastBlink_w = 0;
volatile unsigned long lastBlink_g = 0;
volatile unsigned long watt = 0;
volatile unsigned long waterflow = 0;
volatile unsigned long gasflow = 0;
unsigned long oldPulseCount_e = 0;   
unsigned long oldPulseCount_w = 0;   
unsigned long oldPulseCount_g = 0;   
unsigned long oldWatt = 0;
unsigned long oldWater = 0;
unsigned long oldGas = 0;
double oldKwh;
double oldwater;
double oldgas;
unsigned long lastSend;
MyMessage wattMsg(CHILD_ID_e,V_WATT);
MyMessage kwhMsg(CHILD_ID_e,V_KWH);
MyMessage pcMsg(CHILD_ID_e,V_VAR1);
MyMessage flowMsg(CHILD_ID_w,V_FLOW);
MyMessage volumeMsg(CHILD_ID_w,V_VOLUME);
MyMessage lastCounterMsg(CHILD_ID_w,V_VAR2);

MyMessage flowMsg(CHILD_ID_g,V_FLOW);
MyMessage volumeMsg(CHILD_ID_g,V_VOLUME);
MyMessage lastCounterMsg(CHILD_ID_g,V_VAR3);


void setup()  
{  
  // Fetch last known pulse count value from gw
  request(CHILD_ID_e, V_VAR1);
  request(CHILD_ID_w, V_VAR2);
  request(CHILD_ID_g, V_VAR3);

  // Use the internal pullup to be able to hook up this sketch directly to an energy meter with S0 output
  // If no pullup is used, the reported usage will be too high because of the floating pin
  pinMode(DIGITAL_INPUT_SENSOR_e,INPUT_PULLUP);
  pinMode(DIGITAL_INPUT_SENSOR_w,INPUT_PULLUP);
  pinMode(DIGITAL_INPUT_SENSOR_g,INPUT_PULLUP);

  attachInterrupt(digitalPinToInterrupt(DIGITAL_INPUT_SENSOR_e), onPulse_e, RISING);
  attachInterrupt(digitalPinToInterrupt(DIGITAL_INPUT_SENSOR_w), onPulse_w, RISING);
  attachInterrupt(digitalPinToInterrupt(DIGITAL_INPUT_SENSOR_g), onPulse_g, RISING);
  lastSend=millis();
}

void presentation() {
  // Send the sketch version information to the gateway and Controller
  sendSketchInfo("Marcel Meterkast", "1.0");

  // Register this device as power sensor
  present(CHILD_ID_e, S_POWER);
  present(CHILD_ID_w, S_WATER);
  present(CHILD_ID_g, S_GAS);
}

void loop()     
{ 
  unsigned long now = millis();
  // Only send values at a maximum frequency or woken up from sleep
  bool sendTime = now - lastSend > SEND_FREQUENCY;
  if (pcReceived && (SLEEP_MODE || sendTime)) {
    // New watt value has been calculated  
    // electricity first
    if (!SLEEP_MODE && watt != oldWatt) {
      // Check that we dont get unresonable large watt value. 
      // could hapen when long wraps or false interrupt triggered
      if (watt<((unsigned long)MAX_WATT)) {
        send(wattMsg.set(watt));  // Send watt value to gw 
      }  
      Serial.print("Watt:");
      Serial.println(watt);
      oldWatt = watt;
    }
    // send water
    if (!SLEEP_MODE && waterflow != oldWater) {
      // Check that we dont get unresonable large watt value. 
      // could hapen when long wraps or false interrupt triggered
      if (waterflow<((unsigned long)MAX_WATER)) {
        send(wattMsg.set(waterflow));  // Send watt value to gw 
      }  
      Serial.print("Gas:");
      Serial.println(gasflow);
      oldWater = waterflow;
    }

    // send gas
    if (!SLEEP_MODE && gasflow != oldGas) {
      // Check that we dont get unresonable large watt value. 
      // could hapen when long wraps or false interrupt triggered
      if (gasflow<((unsigned long)MAX_GAS)) {
        send(wattMsg.set(gasflow));  // Send watt value to gw 
      }  
      Serial.print("gas:");
      Serial.println(gasflow);
      oldGas = gasflow;
    }
    
    // Pulse cout has changed
    if (pulseCount_e != oldPulseCount_e) {
      send(pcMsg.set(pulseCount_e));  // Send pulse count value to gw 
      double kwh = ((double)pulseCount_e/((double)PULSE_FACTOR_e));     
      oldPulseCount_e = pulseCount_e;
      if (kwh != oldKwh) {
        send(kwhMsg.set(kwh, 4));  // Send kwh value to gw 
        oldKwh = kwh;
      }
    }
    if (pulseCount_w != oldPulseCount_w) {
      send(pcMsg.set(pulseCount_w));  // Send pulse count value to gw 
      double water = ((double)pulseCount_w/((double)PULSE_FACTOR_w));     
      oldPulseCount_w = pulseCount_w;
      if (water != oldwater) {
        send(kwhMsg.set(kwh, 4));  // Send kwh value to gw 
        oldwater = water;
      }
    }
    if (pulseCount_g != oldPulseCount_g) {
      send(pcMsg.set(pulseCount_g));  // Send pulse count value to gw 
      double gas = ((double)pulseCount_g/((double)PULSE_FACTOR_g));     
      oldPulseCount_g = pulseCount_g;
      if (gas != oldgas) {
        send(kwhMsg.set(gas, 4));  // Send kwh value to gw 
        oldgas = gas;
      }
    }    
    lastSend = now;
  } else if (sendTime && !pcReceived) {
    // No count received. Try requesting it again
    request(CHILD_ID_e, V_VAR1);
    request(CHILD_ID_w, V_VAR2);
    request(CHILD_ID_g, V_VAR3);
    lastSend=now;
  }

  if (SLEEP_MODE) {
    sleep(SEND_FREQUENCY);
  }
}

void receive(const MyMessage &message) {
  if (message.type==V_VAR1) {  
    pulseCount = oldPulseCount = message.getLong();
    Serial.print("Received last pulse count from gw for:");
    Serial.println(pulseCount);
    pcReceived = true;
  }
}

void onPulse_e()     
{ 
  if (!SLEEP_MODE) {
    unsigned long newBlink = micros();  
    unsigned long interval = newBlink_e-lastBlink_e;
    if (interval<10000L) { // Sometimes we get interrupt on RISING
      return;
    }
    watt = (3600000000.0 /interval) / ppwh_e;
    lastBlink_e = newBlink_e;
  } 
  pulseCount_e++;
}

void onPulse_w()     
{ 
  if (!SLEEP_MODE) {
    unsigned long newBlink = micros();  
    unsigned long interval = newBlink-lastBlink_w;
    if (interval<10000L) { // Sometimes we get interrupt on RISING
      return;
    }
    waterflow = (3600000000.0 /interval) / ppwh_w;
    lastBlink_w = newBlink;
  } 
  pulseCount_w++;
}

void onPulse_g()     
{ 
  if (!SLEEP_MODE) {
    unsigned long newBlink = micros();  
    unsigned long interval = newBlink-lastBlink_g;
    if (interval<10000L) { // Sometimes we get interrupt on RISING
      return;
    }
    gasflow = (3600000000.0 /interval) / ppwh_g;
    lastBlink_g = newBlink;
  } 
  pulseCount_w++;
}