I've recently went to triple monitor as well with XFire 6870's. I was surprised the I can run BF3 with this setup on medium settings. I do plan on upgrading to a 7950 at some point, though.
So, chances are that XFire's working fine, just that these cards aren't the greatest for the setup. GPU-Z will definitely tell you for sure.
To swap from EyeFinity to a single, just make 2 different profiles in CCC. That's what I do.