Kbase Article of the Week: Mac Pro (Early 2009), Mac Pro (Mid 2010), Mac Pro (Mid 2012): Issues with Three Displays and Multiple DVI, HDMI Connections →

This kbase article includes some real word spaghetti:

The Mini DisplayPort to DVI Adapter (different from the Apple Mini DisplayPort to Dual-Link DVI adapter) is considered a single-link, passive DisplayPort device. However, you should use only dual-link adapters (also known as “active DisplayPort” adapters) with multiple display configurations. Single-link DVI to Mini DisplayPort (or VGA) adapters may not work as expected in multiple display configurations.

If you use three displays, you should connect at least two of them with a Mini DisplayPort. You can also use the Apple Mini DisplayPort to Dual-Link DVI adapter or the Apple Mini DisplayPort to VGA adapter or just connect the devices to the graphics card using their native Mini DisplayPort connectors.