well, it depends. for some ram will revert back to single channel so that it can work, and some ram will annoying and not do that. that mobo is meant for triple channel ram, so i would get triple channel.
First, make sure the RAM is compatible with the board; check RAM voltages (usually on the sticker on each RAM module) against the board's RAM voltages. If they are compatible, and the RAM modules are the same model and versions (same latencies and voltages), why not use three? Then you have a spare.