Using the latest drivers, I get an abysmal frame rate of 16 - 20 FPS (with the latest patch) -- this is also using the default SLI bits that come with the Witcher 3 SLI profile (0x080020F5). There has been fixes from users on the forum (the user named Helifax takes credit for this particular work around) that have found that if you plug in all three displays to a single GPU (while still using SLI), The Witcher 3's frame rate suddenly increases. I myself do not have $99 to spend on aan active DVI->DisplayPort adapter, so I cannot use this work around as a means to play The Witcher 3 with nVidia surround with reasonable frame rate. SO, the other work around that the community has found is my only solution. However, this involves replacing the default SLI bits with the ones of the Assassin's Creed: Rogue profile (0x0A0040F5). These bits increase my FPS to around 35 - 45 FPS, even getting up to 50 FPS at some parts. However the lights in the game flicker and flash about so badly, as though the game was undergoing some strobe light effect, making the game unplayable.

EDIT: So I booted it up and yeah, it has poor scaling in 2+1 config. Try changing your outputs to have all monitors on one card while running surround, it will probably work. BUT a lot of games don't seem to like that config - Mad Max and battlefield 4 for example, produce artifacting. A few games like to crash (although with the current state of drivers, who knows). But the normal guidelines are to run 2+1 when you have Sli.