I don't recommend even 2xSLI.
Many games don't support it, and near-future games are even LESS LIKELY to support it as well. WHY?
Modern game engines look for ways to optimize things. One way is to find SIMILARITIES between frames (which is how video COMPRESSION WORKS). This requires the same GPU to work so it does not work with the typical AFR implementation of SLI.
That is why Batman AK could not support AFR. (Alternate Frame Rendering, which most people just call SLI since there's only a couple games that do other methods than AFR such as SFR)
For games that do work there may be added STUTTER/JUDDER or other issues.
I'm NOT saying it's completely crappy, just that I think it's more hassle than it's worth for one thing, and IMO not even worth the cost.
*I would recommend selling the GTX1070 and buy a GTX1080Ti instead. (or wait for another, SINGLE card that's better value)
It's probably going to be at least TWO YEARS before we see a switch towards SFR (Split Frame Rendering) working on multiple GPU's. There's little incentive, especially since again there are single-GPU optimizations being done.
(I do think there's a strong chance of multi-GPU in the PS5 and next XBOX though. Heck, the PS4 PRO has a dual-GPU but we need the new generation of consoles to be multi-GPU to really push that into PC gaming more heavily)