Good article here on SLI:
https://www.pcworld.com/article/3071332/hardware/its-true-nvidias-geforce-gtx-1080-officially-supports-only-2-way-sli-setups.html
Nvidia supports 2 way SLI but no longer supports 3 or 4 cards in SLI configuration.
As pointed out, even that is irrelevant with ED not supporting even 2 way SLI. I had 2-780's at one point and it gave no value at all. Crysis, hell yeah, but not in DCS.
At this point, I would say that 2 way SLI is more than enough if it were supported given the current inability to utilize multi-core CPU's. If we had both, life would be grand.