I just got a third monitor and a second graphics card. All three monitors are identical (Acer 20" something or others) and the graphics cards are identical Nvidia 550TI cards. A few things I cannot seem to figure out: how do I get the task bar to span across all three screens, and how in the hell do I get games to span across all three screens. According to Nvidia, I have to create a custom triple wide resolution using the nvidia control panel and then select that from the game's menu... well that is all fine and dandy, except that the custom resolution has not been available in the games I've tried (so far Skyrim and Killing Floor). If anyone knows of a third party program or the trick to get this to work natively, I would love to know.
NVidia calls it Surround. You will need to plug two monitors into the first card and the third monitor into the second card. Make sure that you have the SLI bridge enabled and that your motherboard supports this. You will then need to make sure that you have the NVidia drivers installed.
You will want to check with the game manufacture and/or forums to see if people have issues with NVidia Surround and your game.
Widescreen Gaming Forum is a nice website that handles a lot of issues that you may be experiencing and also taking it a step further to optimize the game.