My setup:
1 DVI, 1 HDMI (to VGA adapter), and 1 Display Port(to HDMI adapter).
I would be fine with the HDMI and DisplayPort monitors displaying the same image.
AMD Catalyst Control Center's Desktop management knows of the Display Port display but says that another needs to be disabled for it to be activated.
Windows screen resolution settings fails to save any settings that use the Display Port monitor.
Best Answer
You probably are using a passive DP to HDMI adaptor, that is a common issue. Most AMD cards can output to two monitors maximum using the HDMI/DVI (it's the same type of signal) signal. The way a passive adaptor works is it outputs the HDMI/DVI signal but through the DP port and the passive adaptor literally is just metal lining up certain pins of the DP cable with certain pins of the HDMI cable. If you want to have 3 monitors you will need to get an active DP to HDMI converter which actively reads the DP signal and translates it to an HDMI signal. It may also be possible to get a passive DP to VGA adapter, I don't know about that for sure since I have never used a VGA monitor with my DP ready cards.