Multi-card setups using SLI and CrossFire have always been in the minority, but the community remains vibrant enough that NVIDIA and AMD continue to support them. When you venture into triple and quad-card configurations though... that's when the waters get understandably rougher.
Fortunately, the release of NVIDIA's Pascal chips could mark the shift from relying on GPU vendors for support and instead putting the onus on developers.
PC Perspective's Ryan Shrout recently contacted NVIDIA regarding its stance on multi-card setups using the company's latest hardware -- the GTX 1070 and 1080.
While NVIDIA doubled-down on its commitment to dual SLI, its comments on additional card configurations was intriguing (emphasis mine):
DX12 and NVIDIA VR Works SLI technology also allows developers to directly implement and control multi-GPU support within their games. If a developer chooses to use these technologies then their game will not need SLI profiles. Some developers may also decide to support more than 2 GPUs in their games. We continue to work with all developers creating games and VR applications that take advantage of 2 or more GPUs to make sure they’ll work great on GeForce 10-series GPUs.
One of SLI's weaknesses has been the reliance on driver profiles supplied by NVIDIA in order to best utilise the powerful setup in games. However, with the rise of DirectX 12, which provides more flexible and direct access to graphics resources -- including the mixed-vendor, multi-GPU mode -- it's entirely possible you'll need to badger developers rather than the GPU manufacturer if you're running a multi-card setup.
Three and four-card SLI configurations will never be the norm, of course, but it'll be interesting to see how this new dynamic plays out in the next few years. It could work out especially well for indie VR titles that require the grunt and aren't able to get the attention of NVIDIA for specialised support.