I have yet to see any rig capable of running 3x4k monitors, unless you are planning to go something along the lines of four liquid cooled 980ti's or other levels of insanity.
Not that I'm not envious if you can afford it.
Personally I would rather look towards a singel 3360:1440 21:9 at about 36" it should do quite nicely and you don't need to sell half your liver and a kidney to pay for the rig to run it

That and multi screen gaming just brings to many bugs with it, I have never managed to get mine to fully synch and 16:3 aspect ratio just bugs out the fov on most games so even though I have had a trippel screen setup for almost six years I prefer gaming on the center monitor anyways, having another two for Netflix or whatnot is quite handy with games like Elite.
Anyhoo that's just my nickel.

Hey I'm Thor -
People call me Bob.
Rule 1: Pillage. Then burn.
Rule 2: No such thing as overkill, as long as there are reloads.