For that you'll need to "Extended" you second and third displays and designate your center as the Primary Display. Then you can fling the Steam to one side and Firefox to the other, and game on the center. Of course, this will mean your Eyefinitiy resolution will be disabled. But you can also just run your Eyefinity reslolution on a game, and size the other programs to fit on your side displays. Windows 7 is pretty good with remembering previous window size/position, even after swapping screen setups.
a tip for ya: if you decide to have all monitors as seperate displays (no Eyefintiy), I suggest staggering the edges so you can still move you mouse to the edge to bring up the taskbar, sidebars, etc. On mine, i have only about 10% of each display touching, as such:
The asterisk denotes the Primary display.
If course, once you have it set up, be sure to save the profile so you don't have to go through the fuss of lining them up again. A warning: with most ATI driver versions, I've ahd to load Extended profiles TWICE for it to align the screens properly. I've also noticed that a game or two that run like shit with the extended displays enabled, pictured above. Disabling the side display fixes it.