All Intel Core CPU's that have 16 PCIe lanes can only support a 2 way SLI. This is because Nvidia requires a minimum of 8 PCIe lanes per video card. And you cannot split 16 into 3 pieces and still have each piece have 8 or more. Beware of motherboards that do 16x/0x, 8x/4x/4x but not 8x/8x/0x. Those boards will not SLI at all.
Before we get too far along, triple SLI is where most "money doesnt matter" people stop because the return on investment keeps dropping with each card added to SLI or Crossfire. 2 cards generally add 75%+, 3 cards add 45%+, 4 cards add 15% to 25%.
If you want to go 3 way or 4 way SLI, you need one of three things.
1) X99 motherboard and CPU. Lowest end X99 CPU's come with 24 PCIe lanes, and as long as they do not take even a single PCIe lane for something else, then you can do 3 way SLI. But if they take even 1 PCIe lane for something else, you will be stuck with a maximum of 2 card SLI again. The 6 core X99 chips come with 40 PCIe lanes. These are the CPU's where you can do SLI in 3 way or 4 way.
2) Buy a pair of Dual GPU cards. Each of these cards is an SLI unto itself, and then you can pair these up.
3) There are a small number of motherboards out there that use Plex switching. Plex switching uses a special chip on the motherboard which takes certain numbers of PCIe lanes, and shares them between slots. These boards are not cheap. And they seem to be fairly rare. I just looked around... I found one...
Brand
MSI Z97 XPOWER AC
LGA 1150
Core i7 / i5 / i3 / Pentium / Celeron (LGA1150)
PCI Express 3.0 x16
5 x PCIe 3.0 x16 slots (PCI_E1, PCI_E2, PCI_E3, PCI_E5, PCI_E7)*
- The operating mode will be x0/x16/x0/x0/x0, x16/x0/x0/x16/x0,x16/x0/x0/x8/x8 or x8/x0/x8/x8/x8.*
- Support for installing up to 4 PCIe x16 expansion cards.
Its not cheap. $369.99 @ Newegg. I also will warn you that the board has a 3 egg (star) average at Newegg. But there have only been 11 comments.
In looking at the motherboard and the way the slots are lined up, if you go with 4xSLI, the 4th card would be hanging out over the end of the motherboard, so think about that when ordering a case for this monster motherboard and her 4 video cards.
http://www.newegg.com/Product/Product.aspx?Item=N82E16813130764
There may well be others out there. I just don't have the time today to keep digging. Just do a google search for "Z97 motherboard Plex" and it should find them for you.
I would also recommend that you read some reviews on these motherboards that have the plex chips. While they do their magic well, they also add some latency, and that is not always a good thing. Just understand what you are getting, and if the plex chips are likely to affect you negatively, move on up to an X99 motherboard and CPU with 40 lanes of PCIe. One other note... X99 systems require DDR4 memory, which is new, which means it costs a whole bunch more money than DDR3 would.