2 monitors per videocard is what you should look for. If you want three monitors, you only need two videocards. Would suggest running the main monitor from one and the two extra's from the other.

Either that or a HD3870x2 might do the trick
http://www.azerty.nl/_azerty/data/pr...00/img/1/G.jpg

I'd go for 2x decent cards primarily tho. :P