Question: When I bought my baseboard heaters, thermostats and 220V breakers I was advised to get 12/3 wire. When I attempted to install the heaters I found that the thermostats, heaters, and even breakers have connections for only 2 wires. I have since read that 12/3 is only necessary for wiring something like a dryer that uses both 220V and 110V over the same wire. My question now is, should I get 12/2 instead of 12/3 to wire these heaters?

Answer: 
You should use 12/2 if the 240v appliance requires two hot plus ground. If you can't find 12/2 wire that has black/red/ground... you can use the more common wire that has black/white/ground where both black and white are hot. If doing this you MUST identify the white wire as hot at the appliance (or outlet if used) and at the panel. I always use colored heat shrink tubing because electrical tape almost always falls off after many years and is not reliable. Also make sure that you're using a proper 2-pole breaker at your electrical panel and if using an outlet, use the correct type to match the circuit's amp rating.

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EDIT: As comments have noted, this isn't a good answer. Thanks for the lesson! Look at the other answers on this page for wiser advice.

12/2 would be a less expensive and more obvious-to-future-electricians way to wire the heaters. However, you can use 12/3 and just ignore one of the wires. The safe way to do that is to bond the red wire to ground at every junction. That way, if it is ever wired to current somewhere in the system, the circuit will immediately short and your circuit breaker will break the circuit.