h2 = dht2.readHumidity(); // Sensor readings may also be up to 2 seconds 'old' (its a very slow sensor)
t2 = dht2.readTemperature(); // Read temperature as Celsius (the default) // Read temperature as Fahrenheit (isFahrenheit = true)
f2 = dht2.readTemperature(true);
if (isnan(h) || isnan(t) || isnan(f)) {
Serial.println(F("Failed to read from DHT sensor 2!"));
return;

if (isnan(h) || isnan(t) || isnan(f)) {
should be h2, t2, f2, and the same for the third DHT

When using BME280 breakout https://lowpowerlab.com/shop/product/185 with Embedded Adventures library, life is easier if addrBME280 is set to 0x77 (it is set to 0x76 in BME280_MOD-1022.h)

h2 = dht2.readHumidity(); // Sensor readings may also be up to 2 seconds 'old' (its a very slow sensor)
t2 = dht2.readTemperature(); // Read temperature as Celsius (the default) // Read temperature as Fahrenheit (isFahrenheit = true)
f2 = dht2.readTemperature(true);
if (isnan(h) || isnan(t) || isnan(f)) {
Serial.println(F("Failed to read from DHT sensor 2!"));
return;

if (isnan(h) || isnan(t) || isnan(f)) {
should be h2, t2, f2, and the same for the third DHT

You can calibrate a humidity sensor for temperatures around "room temperature" (25°C / 77°F) using the standard salt test, please see https://www.allaboutcircuits.com/projects/how-to-check-and-calibrate-a-humidity-sensor

Numbers in spec sheets are often provided for some standard EPA conditions ("National Primary and Secondary Ambient Air Quality Standards", 40 CFR—Protection of the Environment, Chapter I, Part 50, Section 50.3, 1998), 25°C / 760 mmHg. From my experience, at high humidity values (70+%) and low temperatures (10..12°C) grey market Si7021 sensors can overestimate humidity as much as up to 10%. I have no experience with HYT221, but suspect that the phrase "The module is precisely calibrated by the manufacturer and is therefore fully interchangeable without adjustment" in https://www.ist-ag.com/sites/default/files/AHHYTM_E.pdf is valid only for the "official" sensors, not to any grey market ones.