from what I know, you can really only run 2 cards as video cards, using the 3rd card as a Physics card...for Quad SLI you need 2 7950 Gx2 which each have 2 cores....
(2 cores x 2 cards in SLI =
I heard some rumors about ATI comming out with "trifire" using all 3 GPU slots on some mobos (pci-e or pci-x)
but then again, who wants ATI? the nVidia guy clearly PWN3D the ATI guy and the ATI guy got crushed first.