I just got a new PC with a GTX 970 video card. It has a DisplayPort, HDMI and 2 DVI ports. When I plug a monitor into the DisplayPort and one into a DVI port, they work perfectly. When I plug another monitor into the second DVI port, the one plugged into the first DVI port is disabled (doesn't receive a signal any more). From what I learned from google, this card is supposed to support 3 monitors simultaneously.
I'm running Windows 10, with the driver downloaded from GeForce Experience (version 355.60).
I just noticed that while booting, the only active monitor is the one that's disabled while in Windows. So the BIOS etc shows on monitor 2, while monitor 1 and 3 become active instead when Windows starts.
Best Answer
It turns out the NVidia Configuration utility was the culprit. It disabled the second monitor once I connected the third, causing it to be unavailable to the standard Windows monitor configuration. There was no mention from the utility saying it disabled the monitor. Enabling the monitor in the NVidia utility solved the problem.