I have one 20 amp double pole circuit that I am planning to run 3 baseboard heaters on with 2 line voltage thermostats. I have already made sure the watt consumption for the 3 heaters will not exceed 3,840 watts. I have run 12/2 wiring from the breaker to the first thermostat location which will control 2 baseboard heaters and from there to the other thermostat which will control the 3rd heater. I have uploaded a quick diagram for visual reference. I believe I know how to correctly wire this up but just want a sanity check before committing. I believe I can just pig tail at thermostat 1 to pass power through to the second thermostat location (bedroom) and for the thermostat that controls two baseboard heaters, it seems like the wiring is simple. Any help is appreciated!
You are correct -- just don't overstuff the box!
Pigtailing the wires to room 2 with the line-side of thermostat 1 will get you what you are after here. However, with the thermostat and the 3 12/2s in there, that's too much stuff for many single gang boxes (20.25 in3 of box fill vs the largest standard device box in the Code being 18in3), so you'll want to use a bigger box such as a 4" square with a single gang cover/mudring there instead. You'll also want to wrap each of the white wires in this with black tape to make it clear that they're hots, not neutrals.