I found this post when I was looking to connect to a _fourth_ monitor and figured I'd share my scenario. I have a Macbook Pro screen, a separate monitor using my miniDisplayPort, and I use my iPad's "Air Display" app as a third monitor. I was looking to see what it would take to setup a fourth. Thanks for the insight on using the Thunderbolt and/or USB port. I will try that, perhaps both since I'm a nerd, and let the thread know which I found to be the best.
Note on using the "Air Display" app: the refresh isn't lightning fast so you'll probably see a slower refresh than you would even on the Thunderbolt or USB port. That being said, I keep my email and other static office apps on my iPad screen using "Air Display". It's a great way to add extra desktop space if you already have an iPad. In fact, I highly recommend "Air Display". I don't usually push apps on other people but I figured many in this thread (and perhaps others who come here later) would benefit from knowing about it. Here's their main website followed by the iTunes page for it. For $10 you really can't beat it. The setup is easy.
http://avatron.com/apps/air-display
http://itunes.apple.com/us/app/air-display/id368158927?mt=8
Have a good day everyone!
AG3