Yes you do, make sure the cards have a high memory capacity too, if you decide to go the three monitor route, a 3 gigabyte AMD Crossfire would be necessary. Or you could go with a 4 gigabyte GTX 670 SLI for example. Remember that these memory amounts don't combine to give you 6 or 8 gigs of memory. Both memory modules have to load the same textures, so you still only have 3 or 4 gigs depending on your card set up. (3 for amd, potentially 4 for nvidia).
As for required gpus, i think 2 7970s would be decent for 3 monitors. Or you could go with two 4 gigabyte 680s if you had the money for it. I hope i helped