and then have only heater1 kick in when outside temp is 5’ or less below set temp, heater 1 and 2 if outside temp is 10 ’ or less below set temp, and 1, 2 and 3 if outside temp is more than 10’ below set temp
Until this FR is implemented (although I doubt it ever will) let me know if you want a way to achieve your goal now (using Generic Thermostat and a Template Switch).
Create a Template Switch named switch.heaters like the following example. Be sure to change climate.your_thermostat and sensor.your_outdoor_temperature to the actual entities you are using.