I've tried two and three video cards and I couldn't get them to work right together. I bought the Matrox Triple Head to Go Digital and everything works as I had wished. Makes it so much easier to use the three monitors together. Good for racing sims but a little hard to keep all that real estate covered in a first person shooter.
The Triple Head To Go is $300. I'm running triple sli, 3 x 8800gtx, but I think that's overkill. I just wanted to try it Matrox website should give you more information about video card requirements and what surround (wide) resolutions to expect. then get three identical monitors or at least monitors with the same native resolution. I had tried all other ways but never got it to work right until I got the triple head to go.