1) RESOLUTION vs FPS is not directly comparable when the RATIO is different... the WIDER the resolution the LESS the extra processing relative to pixel count (because there's less going on in the sides).
3x1080p is probably going to get you about 2x the FPS that 4K would do (though it varies a lot)
2) 3x1080p??
I really dislike triple-monitor gaming as there's a bezel gap as well as the difficulty in properly supporting that ratio. It's also still pretty low res horizontally (1080).
3) *I would much rather have something like this:
a) 3440x1440@100Hz (GSYNC), or
b) 1x2560x1440 (GSYNC) + 2x secondary monitors (for non-gaming)
4K is currently problematic for gaming since the refresh rate is low (i.e. 60Hz). There are 2560x1440 monitors that support 165Hz and actually the resolution scales better with many programs too. 4K still has some scaling issues where DPI is larger but blurry and 4K is too small (may be a non-issue for most. I don't know).
4) SLI STUFF:
SLI has different parts. What we really mean typically is AFR or Alternate Frame Rendering. That's where GPU#1 processes a frame, then GPU#2 does the next frame.
Games are becoming harder to code towards AFR so some modern ones have no support at all, and those that do may have poor scaling, added stutter, or both (you also add lag due to the buffering).
So I do NOT recommend SLI for gaming, especially if you have a GTX1080Ti already.
5) System?
First, I'd work BACKWARDS from the desired monitor. It's roughly $1000USD for a good 3440x1440, GSYNC monitor:
https://pcpartpicker.com/product/WDcMnQ/acer-monitor-umcx1aa002
after that I'd look at the CPU but without knowing specifics here's a very quick parts overview:
i7-8700K
Noctua NH-D15S (OC CPU to 5GHz or close... not pretty fan but I dislike liquid coolers due to pump, reliability etc)
suitable motherboard
16GB 3200MHz (2x8GB) DDR4 CL16
GTX1080Ti Asus Strix or similar
I might even set a Global Cap to 95FPS to stay in GSYNC MODE at all times (assuming 100Hz).
Do note that MULTIPLE MONITOR setups can have different problems. It's not as bad as it used to be but I personally would prefer:
a) same exact refresh rate, and
b) all IPS panels to calibrate color, and
c) same HEIGHT
I'd actually prefer everything to be IDENTICAL but I've seen situations where turning the Monitors 90degrees is handy for tasks where height matters more than width (so middle is 16:9, and side monitors are essentially 9:16 but the "height" still lines up physically).
*With TRIPLE MONITORS you also need to either turn your head constantly (bad) or push the monitors further back which defeats the purpose of having larger monitors... I only recommend Triple monitors for productivity people but even then I'd probably be looking at a single 4K, 32" screen instead to have 4x1080p zones if needed, or maybe 3440x1440.
Are a few good videos on 3440x1440 vs 4K for productivity though for gaming AND productivity I'd go back to these specs as the best compromise:
3440x1440
IPS
GSYNC
100Hz