In the end the X58 chipset only has 36 lanes for PCIex, so none will be true 16/16/16. Most are setup to run 16/8/8 or 16/16/4. The Asus P6T, P6T Deluxe, P6T WS, and Rampage II Extreme all have it. DFI's T3eH6, Gigabyte's UD4P, UD5, and Extreme do as well. Other brands can do it as well, but your best options are those listed above. Since it's not possible to run 16/16/16, you might be better off running 2 powerful cards in SLI and a 3rd, cheaper card as your PPU. IE. GTX285/GTX285/9800GT (16/16/4). The benefit for the 3rd card in SLI is less than the benefit from the 2nd, since there aren't enough lanes for the card to deliver at full tilt.
No, you can SLI 2 cards of the same model, and then use the 3rd different card as a PPU (Physics Processing Unit utilizing nVidia's PhysX api). For games that have PhysX support, this offloads alot of work from the CPU, UnrealTournament 3, Mass Effect, Cryostasis, and Mirror's Edge are a few that use PhysX. You can check out a list of the games supporting it here: http://en.wikipedia.org/wiki/PhysX
Any Geforce card that supports CUDA can handle PhysX (so 8000 series and higher).
Alternatively, you can enable PhysX on your GPU or GPU's in SLI rather than have a dedicated card just for PhysX.
GTX260s and GTX285s both support 3 way SLI as well. GTX295 supports 2 way SLI (= Quad SLI, since they have 2 GPUs each). In addition to the 9800GTX, the 8800 Ultra and 8800GTX support it as well. All that aside, with the Core i7 platform, the GPU becomes the bottleneck, so going with anything less than GTX260s would be a waste IMHO.
From Gigabyte's website:
''2 x PCI Express x16 slots, running at x16 (PCIEX16_1/PCIEX16_2) (Note 3)
1 x PCI Express x8 slot, running at x8 (PCIEX8_1) (Note 4)
(The PCIEX16_1, PCIE16_2 and PCIEX8_1 slots support 2-Way/3-Way NVIDIA"
"(Note 4) The PCIEX8_1 slot shares bandwidth with the PCIEX16_2 slot. When PCIEX8_1 is populated with a PCI Express graphics card, the PCIEX16_2 slot will operate at up to x8 mode."
As I said before, its a limitation of the chipset, X58 only has 36 lanes for the PCIe bus. 16/16/8 would be 40 lanes, therefore, not possible. The NewEgg listing and GigaByte's description is misleading, they make it sound like it can run 16/16/8