You can do it with 2 vid cards, and hook 2 monitors up into 1 card, and the third into the second card.
Nvidia drivers have this feature (outside of windows) where it treats all monitors as a single resolution desktop. I played around with it with my 2 monitors in HL2 to see how it worked. Windows thinks it's all 1 monitor, so it's all driver-side (whatever "it" is doing).
I'm pretty sure it would work with 2 vid cards and 3 monitors as well.
Barring that, Matrox triple-head, but IMO that's kind of expensive, and you need quite a video card to put out those pixels to begin with.
