For three cards you are within the realm of wanting 2011 platform. You could find try and find a motherboard that is 8x/8x/8x but I think all of the triple card boards use the NVidia chip for additional lanes. They aren't exactly the perfect solution but they do restore some of the performance difference of having less real lanes.
With 2011 at least you would have 16x/8x/8x natively and given PCI-E 3.0 you would have no performance degradation at all. But conversely the CPU is going to be slower and the platform is over a year old. No native USB 3.0 for example.
Its not an easy decision actually, there are performance trade offs. I would probably take 2011 instead of 1155 but then I can find a use for those 2 additional cores.
i7 3930k @4.4, 2xMSI GTX 680, 16 4x4 GB Corsair 2133 RAM, Crucial m4 500GBXonar D2X. All custom watercooled by 2x MCR 320 and 1 MCR 480
Zowie Evo CL EC2, Corsair K70, Benq XL2411T