If it is FPS games the last thing you want it to try and get it to span across 2 monitors, not much good when the crosshair ends up split on both monitors.
As far as using 3 monitors, unless you have a HD 5xxx series card or 2 Nvidia 4xx series in SLI (don't think they have added support for it to the GTS/GTX 2xx series cards yet), you will need something like the Matrox TripleHead2Go to be able to use 3 monitors.
For a lot of the newer games they are starting to support 3 monitors or have support through the video card drivers
. A good place to start with seeing what works and what doesn't is here, Essential Games List - WSGFWiki