This should explain Crossfire.
As for Crossfire in theory it doubles the performance, So with 3 screens in tri Crossfire you would have equivalent of 1 per screen. You will need to take into account your PCIe bus, if you go for newer Crossfire GPUs and your happy with your CPU you'll need to take that and the rest of your system into account. The API in use and its overheads. Games are generally made to run at "ultra" more thinking about CPU bottleneck etc.
Newer Crossfire cards use the PCIe 3.0 bus. Power supply need to be thought about. Monitor what your system is doing in 3DMark FireStrike Ultra, this runs in 4k and will pinpoint where your system may be lacking if it is.
I would recommend an x2 card, 2 chips on one. It saves messing about, 2 power cables as before. There are reports the Fury x2 will be out next year, have a Google on it it might be a worthwhile upgrade providing the price is right. Will last a good many years.