Here's a related question.
Is it possible to run a game at a resolution lower than your monitor's native resolution and still have it take up full screen? The reason I ask is because I couldn't get this work with TW2. And if it is possible then I might run TW3 at a lower resolution to get better performance.
It depends on the monitor, drivers, phase of the moon, your standing with the deity of your choice, things like that.
In TW2, anyway, you can tell the game to use a particular mode and full-screen display. It will try to set the display to that mode. It may succeed, do nothing, or set the display to a useless state (for example, one-third of the display is off the right margin). If using the game settings doesn't help, you may have better luck setting your desktop to the resolution you want and then launching the game.
Either is more likely to work if you are on a DVI or HDMI connection than if you are on VGA (the old 15-pin) connection.