If you are going to use all 3 for gaming, I would suggest sticking with matching size and resolution. It will work mismatched, but it gets annoying when your cursor jumps from one resolution/size to another.
Also keep in mind that the higher spanned resolution is going to cut your frame rate a bit.
Compare your fps with one, or two monitors. More memory, cpu power, more gpu power, and more gpu memory.... Those demo systems have chained multiple high-end video cards. Many games likely won't run as well as we would like on more than one monitor, but if they support amd eyefinity there's a better chance they might, however it might only be eyefinity.