I f you read through some of the benchmarks you can see how the card will scale and it will give you a good idea of the cards frame rate at different resolutions,
http://www.bit-tech.net/hardware/graphics/2014/05/19/amd-radeon-r9-280-review-feat-xfx/4
with eyefinity being approxamately two steps past 2560x1440, ie 1920x1080= 2M pixels , 2560x1440= 3.7M pixels, 5760*1080=6.2M (eyefinity)) you can see that your framerate would dip well below acceptable (likely down ~10-15, if 3gb is sufficient Vram for that game) on ultra, however you may be able to compromise you settings (to med/high) and get back up to atleast 30 fps
OR you could invest in a second 280. but all told, i wouldnt recommend eyefinity on your card