Make sure you have a DVI to Display Port adapter for one of your three screens,
Go into your ATI CCC software,
Find desktop and displays tab,
Right click the drop down arrow on the big screen and create a group,
Now open up your game and select your new monster screen resolution,
You might need to adjust your FoV,
profit.
Be warned, running 3 displays worth of pixels requires a lot of GPU horse power, you'll have to drop your graphic settings down to run a stable 60FPS.