I can provide some insight on this. I currently have 2 Sapphire R9 280x in Crossfire, one is a Toxic, the other is a Vapor-X Tri-X OC. I also have 3 Asus VG248qe monitors. The monitors are connected using the DisplayPort and 2 DVI ports with Dual Link cables, all three monitors connected to the primary card, the Toxic.
This has been nothing short of a nightmare to get going correctly, and Sapphire support hasn't been any type of help at all. Usually I can get 144hz on the display port and one of the DVI ports, but only 60hz on the other display port. For some reason tonight I can only get 144Hz on the monitor connected to the DisplayPort.
However, if I use the Vapor-X as the primary card I can get 144hz across all three monitors, BUT I get BSOD every time I start a game.
Debating on selling both AMD cards and buying a NVIDIA card.