You running AMD hugostiglitz?
With nVidia, you must have IDENTICAL screens to run them in surround. How identical? I mean identical. I tried running 2 Samsung 2333T with one Samsung 2333SW (I literally thought they were the same monitor, just that the T model name was the updated name) and it would not work. I had to purchase a third 2333T and then it worked fine.
Honestly, triple monitors takes serious graphics horsepower. I went 3 monitors thinking my GTX 570's (2 in SLI) would be enough, but I'm just constantly starved for memory. The only graphics cards I would trust to run anything in surround vision would be 3GB 580's. I suppose AMD's new 7000 series is probably fine too, but I'm a green guy so I don't pay much attention to them or know much about them.
Memory is key at huge resolutions. I'm eagerly waiting on Kepler so I can hopefully pick up some 4GB cards sometime this summer and I won't have to worry about memory anymore.