Figuring out electrical connections for THREE electric baseboard heaters with single pole thermostats
Red vs black wires don't matter, you only use red and black because those are the colors chosen when the cable is manufactured. When wiring in conduit, I would use black-black...  They are both hot wires, and they are interchangeable.  

I have no idea why he ran /3 cable, heaters don't need neutral so you normally run /2 cable and remark the white to be black.  


sequence of red vs black on the circuit breaker does not matter.c
both outputs on any given circuit breaker should measure 120V to safety ground, but 240V to each other.  A common mistake with these heaters is using a half-width duplex/twin/double-stuff breaker, which notably does not have the switches ganged, they are independent.  These will have 120V from each leg to ground, but 0V between them. This will not work.  
if you measure between the thermostat contacts, you will see 240V when the thermostat is off.  When the thermostat is on, you will see near 0 volts. 
Nobody cares what color the thermostat switches. 
That is three cables' worth of wire. One cable is from supply, one goes onward to the heater. What is the third doing?  the additional wires go to?  On that subject, what's happening with the white (neutral) wires?