If you didn't have your graphics cards yet I'd recommend switching to NVidia and get the upcoming G-Sync monitor (Asus 2560x1440 120Hz, non-IPS).
4K is a waste of money currently IMO when 2560x1440 is pretty similar for cheaper, with more proven technology.
You don't have to game at those resolutions though (3x4K would be idiotic anyway) so 3x1080p makes far more sense regardless of the native resolution, but use the native resolution for non-gaming. If you are sticking with Crossfire I recommend considering an IPS panel (60Hz) instead as the colour and viewing angles are better. When you turn a monitor on an angle (Eyefinity) you don't want issues there.
*Not what you asked for, but this is what I recommend:http://pcpartpicker.com/part/asus-monitor-pb278q