Eyefinity is a technology included in all of ATI's new 5xxx cards, no other cards have Eyefinity tech. From cheapest/least performance first the options are as follows.
The 5 series cards are quite power conscious too. Your choice should depend on what your wanting to do with the setup, gaming or office work?
There is also the Matrox triple-head-to-go that can power three monitors but grabbing a 57xx is going to be easier/cheaper.
Another thing to consider is that with Eyefinity one of your monitors needs a display port connection.
More info here... http://forums.overclockers.co.uk/showthread.php?t=18086706