Use a latching relay. Here is one I found on Amazon. https://www.amazon.com/dp/B015MRQNXS/?coliid=I3LXTGKX4GEPUR&colid=31WWM91JAUE22&psc=1&ref_=lv_ov_lig_dp_it
If you are trying to use with existing 3 ways use https://www.amazon.com/dp/B015MSYQ8Q/?coliid=I3RP8X9OMYWU97&colid=31WWM91JAUE22&psc=1&ref_=lv_ov_lig_dp_it
Use an optocoupler to indicate lamp status...see the last diagram in this post. https://forum.arduino.cc/index.php?topic=435506.0
@jocke4u I run 2 RF gateways and it's not a problem. When you 'migrate' a node to a new gateway make sure to run the mysensors clear eeprom sketch first to get rid of the old routing table or you might have issues.