It's a pretty simple matter to add a second monitor to your PC. But what about a third? That might require a little more doing.
See, while many desktop video cards offer dual VGA or DVI outputs, it's rare to find one that has three of them. A few squeeze in VGA, DVI, and HDMI, but that would require at least one HDMI monitor. Reader Dave, who asked about this possibility, has three monitors with VGA and DVI inputs only.
You have a few options, Dave. First, you can install a second video card, assuming your desktop has an available expansion slot. Ideally, the second card should be the same make as the first, but that's not mandatory. You can get a reasonably powerful video card for as little as $30.
Next, consider one of the USB adapters I mentioned the other day. Search for "USB to DVI" or "USB to VGA" and you'll find plenty of plug-and-play products that provide a standard monitor input. Prices start at around $50, and graphics performance isn't always great (so this isn't the best choice if you're looking to play games across all three monitors).
One last option is to repurpose a spare laptop or even desktop, using a software solution like MaxiVista to add its screen to your primary system. I wrote about this a while back in Turn a Spare Laptop Into a Second Monitor, but it works just as well for a third monitor. MaxiVista sells for $39.95.