With the purchase of a late 2012 Mac Pro I noticed that I have 3 ports on my video card, two are mini displayPorts and one is a DVI connector. Currently I have two monitors connect via the mini displayPorts and want to plug in a third one using the DVI, I have heard its possible so I got a third DVI monitor a connected it. Once I turned the monitor on, it went black and when I went to my preferences, I noticed that it detected the third one but no image was on the monitor, in fact it stayed in a stand-by mode.
After a little research I realized that you need an active display port adapter to power the monitor signal and by power, I mean it uses a USB port on the back of your mac. So I purchased a Mini DisplayPort to Dual-Link DVI adapter
from Apple in hopes that this would solve my problem. I hooked it up and turned on my Mac to realize that the same problem was happening.
Next I continued to do some research around the internet and came to the conclusion that both Mini DisplayPorts needed to have a powered adapter attached in order for my one video card to support a 3 monitor setup.
So if you want three monitors setup on a Mac Pro, this is how its done.
I should also note that on my PC, I have one ATI card with Eyefinity and it only required me to have one active mini DisplayPort adapter for it to support 3 monitors. Interesting to learn, but glad to have figured it out!