Hi,
Yep you can do it on one & depending on what lvl of detail you feel is ok it will work, also get as much OC out of your card as you can as that will also help. Just remember you card will have a higher idel temp than on a single screen thats normal.
However the power needed to run BF3 on trip screen vs single at what i guess you would run at high/ultra settings on a single screen with that card, you will need about 2.5x / 3x that power to run high/ultra on trip & still get over 60fps, so yep crossfire is a must when it comes to that game & still you may need to turn a few things down, also crossfire does not always scale, but about 95% of the time is fine, but its another factor.
I really like eyefinity its great, but only in the right games, with BF3 it can be a bit much on MP as your taking in so much more information in, but on single player games its really nice or something a little slower MP
However having extended desktop is also great, plain and simple, i would hate only having one monitor now, so there is more + to having trip screens
You can also setup hotkeys to move between Extend & Eyefinity, i have them set to (alt F1 & F2) so its really easy to move once setup.