Can I wire 3 baseboard heaters that total 4000watts and use a 20 amp double pole breaker. I am using 1 themostat for two heaters. And another thermostat for the 1 heater can I do this all with 12/2 wire

Volts X Amps = watts, makes me think this is not going to work.

A 20 Amp 240 volt circuit can only handle 3840 watts, Bal is correct, your plan will not work.

You may install a 30 amp circuit to handle all of the heaters on one circuit, would need a two pole 30 amp breaker and all #10-2 cable to all thermostats and heaters.