If you are referring to the Matrox TripleHead2Go device. It connects to a video card and then drives the three monitors. Al you have to do is set the horizontal resolution to the total of the three monitors.
For example, if you have three monitors that support a horizontal resolution of 1440, then you would set your game horizontal resolution to 3 & 1440 = 4320.
Now, you have to have a video card that can actually drive that type of resolution. The Matrox device takes the one screen image and divides it up to be shown across the three screens when in a game.