None of posts contains correct answer about motherboard capable of 3xSLI. Knowing that X79 and i7 cpus usually offer 40 pcie lanes, it's obvious that motherboard needs additional PCI lanes, which comes as PLX PEX chip.
So the answer would be: any motherboard that has PLX PEX chip; but also there are some intel server chipsets for xeon which might have more pcie lanes.At the time of writing this there is no X99 mobo with PEX chip. Maybe wait a bit until x99 becomes more available.
I would go for ASROCK Extreme 11 motherboard (X79 based mobo) that is capable of 4-way full 16x @ PCIe 3.0 SLI/CF or the fact that it has 8 USB 3.0 ports and much much more is useful to you in any way. Two PEX PLX chips provide sufficient PCIE lanes to PCIe slots and a SAS controller. If you are worried about price, remember that SAS controller costs $300 if you buy it separately. I run 6 SSD drives on that SAS and each drive has very high thruput; this isn't possible with provided Intel chipset SATA controller (transfer speed drops 50% if you have 2 SSDs, and so on). As you see, expensive word is relative. 3x or 4x SLI/CF is backed up by the sufficient PCIe lanes coming out of PEX PLX chips. you are more than safe.
Don't know any other mobo with this characteristics.