This same topic has come up several times.
To play at 4K UHD (3840x2160) - 1 4K monitor - 2x GTX-780 Ti (stock, SC, Classified) or 2x GTX-Titan (vanilla, Black etc.) will suffice.
I am currently running one 4K monitor on my X58 rig w/ 2x GTX-780 Ti Classified GPUs.
In the latest games like BF4 w/ everything on Ultra (no MSAA), I get ~ 100FPS. With 4x GTX-Titan Black SC, I get ~ 170FPS w/ the same settings. With 4x MSAA, the X79 rig is still around 90FPS.
3x GPUs would...