The GTX 1080 only support two-way SLI

With GTX 1080 and GTX 1070, NVIDIA also announced new HB (High-bandwidth) SLI bridge. But this new HB SLI bridge uses both connectors. NVIDIA already showcase the new HB SLI bridges.
GTX SLI HB Bridges

Now EVGA has confirmed that the GTX 1080 not going support anything above than 2-way SLI configurations, no matter what SLI bridge in use. Below is what EVGATech_ChrisB says on EVGA Forum.

“We can confirm that 1080 cards only support up to 2-way SLI, and anything above this will not work, no matter the SLI bridge. 

Please, however, keep in mind that if you do use the new SLI bridge along with two 1080 cards, then it gives more performance than 2-way SLI did in the past on the previous series.”

HB SLI bridge

HB SLI bridge ex

Maybe NVIDIA dropping support for anything over two-way SLI, because it may not be worth the investment in creating SLI profiles and drivers for such small percent of systems. Right now Nvidia didn’t comment on the GTX 1080’s lack of 3-way or 4-way SLI support. But with new HB SLI bridges two cards can give the similar performance of previous 3-way SLI configurations.