Personally I would not even setup a Eyefinity/Surround with out Crossfire/SLI. I think it is a must for best performance. Running three monitors puts a great deal on the GPU. But running at 720p you might be OK I can't say for sure since I have never ran my System at such a low res.
I'm running eyefinity on a MSI R9 280x. I can play battlefield 4 on high settings, and tbh, I don't see much of a difference compared to the max settings (I used to play it on max settings with the same card, but with on monitor). It's also capable of running assassin's creed 4 on max settings, but have a couple filters turned off because they seemed to make the game crash. Don't know if that was a driver or dev error. I bought batman, but haven't gotten around to playing it yet. Also, I believe Skyrim has some issues still with running eyefinity and I can't be bothered to find a fix for that.. Btw, I run those games at 1080p on eyefinity.