I've seen this question about receptacles, but what about thermostats? My house in Washington state has really old mechanical thermostats that don't have their rating specified anywhere, but judging by 12 AWG wire they are 20 amps (all wiring in the circuit is 12 AWG, too, and the breaker is 20 amps). The thermostats I want to replace them with are rated 15 amps. There are total of 3 heaters (in 3 rooms, 1 thermostat per heater) in the circuit, each draw less than 10 amps and total draw should be around 14 amps. Can I use 15 amp thermostat in circuit with 20 amp breaker?
You cannot compare this question to the allowance of a 15A receptacle on a 20A circuit. This is apples to oranges.
You can have as many 15A switches, thermostats, controls, etc, on a 20A circuit, provided the load each item is controlling is within its own rating.
So you can have a 15A thermostat, controlling a 10A load, on a 20A circuit.
The short answer is yes. You can have multiple stats on one circuit. The code requires more than 1 you have 3 so this is up to code. This requires 12awg wiring that you have so you should be code compliant. The OP lives in Washington state. Oregon and Washington have similar limitations on residential circuits. The maximum breaker size on a general purpose branch circuit is 20A. 422.11.E. Single non motor appliance. (2) not exceed 20 amp for loads less than 13.3 amp. (3) Not exceed 150% of rated current. There is an exception for lighting circuits but I have not found it yet. This is why I said more than 1 was OK.