Right now my setup has a nice big 24" flatscreen in the center with a 19" flatscreen to the left. But I have a big gaping hole on the right.
I have a 3rd monitor to put there, but I'm not sure how to get the computer to recognize it. Do I need a graphics card with 3 ports? Can I span the monitors over non SLI-Linked graphics cards? Is it possible to plug my 3rd monitor into the on-board VGA port and have it work?
I'm just migrating to a 3 monitor setup myself. Basically as long as the graphics card works, you can use it. I'm geting 2 Nvidia 8600 PCI-e cards and I'm not using SLI (Motherboard doesn't support it), however SLI is not required.
Almost all operating systems I know of support multiple monitors, as long as the gfx card is enabled, then you can use it.
As for built-in VGA ports, there's no reason that wouldn't work either, though I think most BIOSs disable the on-board display if they detect you've got a gfx card, so you may have to change the BIOS setting so that the onboard VGA is initialised.
Edit: As Will points out, Vista is a bit picky when it comes to multiple cards, all the cards need to use the same driver, so you won't be able to use onboard and another gfx card under Vista.
I would use a second computer and connect the two computers with synergy. Synergy is a software based kvm switch, so it feels like you are using one computer. It's great if like running linux, but you still need a few windows apps. You can keep the windows stuff on the computer with one monitor and have your linux box run the two other monitors. It works on mac too.
Jeff Atwood recently detailed this in his coding horror blog and listed gfx cards with 3 outputs.
An interesting option if you can't add an additional graphics card is to use a USB video card:http://www.evga.com/products/prodlist.asp?switch=10
Assuming you're using Windows XP or Vista, you can use most any combination of video cards
My understanding of Vista is that unless you use old XP drivers, you can now only use multiple cards which share a single set of drivers.
See here: http://www.microsoft.com/whdc/device/display/multimonVista.mspx
I think this is why things like http://www.maxivista.com/ don't really work on Vista.
The Windows way has been covered well. I just wanted to point out that if you can't install another card, you can hook up your third monitor to another computer and use software to "join" the computers. I use Multiplicity from Stardock at work and it works great. It's not free but it's only about $30. It may be an option for you.
You have two options: a Matrox Parhelia (though this can only do three screens if you're using analog rather than DVI), or two graphics cards. My system is running with two PCI Express cards from ATI. These can be SLI linked in a one/two-monitor configuration or can just be used separately (don't count on switching between those modes often, though - when I've done so, driver crashes have frequently resulted). You can probably use the port on the mainboard as an alternative.
One thing that (almost certainly) won't work: digging out that ancient PCI VGA card that you have lying in a box somewhere. They weren't set up for a world of multiple graphics cards and they become very confused by it.
Assuming you're using Windows XP or Vista, you can use most any combination of video cards. SLI is only necessary if you want multiple cards to combine efforts for rendering the same scene.
For example, I'm running two monitors off the dual DVI output from a GeForce 7800GTX and the third is running off a separate GeForce 8500GT. Works great.
In the past, I have run two displays from an AGP video card and the third off standard PCI also.
You can mix cards from multiple vendors, but I've found that things go a lot smoother when you stick with one type of chipset instead. I had (resolvable) trouble a couple of times when mixing drivers for an ATI and nvidia card.
If you're not using the 3rd screen to play video games, you can give one of DisplayLink's USB to video adaptors a try. Just another option if you want to relay video over USB.
Use the builtin Hotkey feature if you are using nVidia cards or purchase a multi-monitor management program. I set hotkeys CTRL-1 (left) CTRL-2 (center) and CTRL-3 (right) for my 3 monitors. I'm also considering switching to a bigger center screen which is currently 20". I think a 30" single monitor works well too.
The matrox triplehead2go is the way to achieve this without extra video cards, especially if you are on a laptop.
My friends and i at the helpdesk i worked at had a similar problem. They provided us with a laptop and 2 monitors with a docking station. The docking station only supported 2 monitors and the OS only supported 2 as well. We wanted to use all three monitors with the laptop screen. So we did some research on how to push video through the USB ports then we found this.
It is a Female HDMI to Male USB adapter user for external monitors. it comes with software that allowed us to use all three monitors. It is plenty fast and its easy to use. They also make a DVI version as well. Hope this helps. Cheers