Eyefinity results in the wrong resolution. 3 1080p monitors result in 5760x1068.
I remember once upon a time it was 5750x1080 like it's supposed to be. I haven't found any settings to manually adjust it. In the Windows display settings it also allows 4536x842 and 3840x712. Maybe it's assuming an incorrect aspect ratio or something?
All three monitors independently display 1080p outside Eyefinity.