In order to get triple channel function, three modules would be placed in the same colored slots. You can run one, two or more modules, your motherboard manual will explain which slots are best to use with various ram module configurations. The problem with using only two modules would be if you wanted to get triple channel function later since mixing and matching ram is not recommended (especially in the same channel). Even if an identical module is purchased later there is no guarantee of triple channel operation. To best insure that your computer will run in triple channel, I will suggest purchasing a matched triple channel kit of ram now - I'd be looking for a 12GB kit personally.
Hope it helps