I found a article on Nvidia's website that should explain it.
Source
"Monitor Prerequisites
In 2D Surround, any combination of monitors that share common timings will work but if the displays are not identical this may result in a lower than optimal resolution, and each must have, at minimum, one DVI, HDMI, or DisplayPort connector. Aesthetically, identical monitors are preferable, but not a requirement.
In 3D Vision Surround, each monitor must be an identical make and model, and you must use three matching digital connectors. We will go into detail below, but you may need additional adapters for this.
The Accessory Display, the forth monitor, does not have to be identical to the three Surround displays, but does require one of the three possible connectors.
Available Ports & Adapters
The GeForce GTX 680 sports two DVI connectors, one HDMI connector, and one DisplayPort connector. In 2D Surround, gamers can use any of the four display connectors to power their Surround setup, and the fourth display connector for the Accessory Display.
In 3D Vision Surround, any monitor connected to the DisplayPort output must use a DisplayPort to Dual-Link DVI adapter so that all three monitors are using Dual-Link DVI. Adapters are available from the NVIDIA Store, Club 3D, and other e-tailers. In future drivers, owners of BenQ’s XL2420T and XL2420TX 3D Vision monitors will be able to use the two DVI connectors and the DisplayPort connector without needing the DisplayPort to Dual-Link DVI adapter, as the BenQ monitors feature built-in DisplayPort 3D Vision technology.
HDMI and DVI adapters are widely available, if required."