Review on 🔌 Cable Matters Active Mini DisplayPort to HDMI Adapter with Eyefinity Technology and 4K Resolution Support by Christopher Buck

This is what you need to install and run your third monitor with a Radeon graphics card. After hours of trying to get it to work with a passive plug this did the trick! Not sure what problems others have had getting it to work with an AMD/Eyefinity card and saying it's not working/active. . Maybe a setting in the Catalyst Control Center? I know AMD has several "certified" connectors listed on their website that I assume should work. I'm using two HD 6800s connected to Crossfire, so I wanted to just use the ports on the main card and not the connected card without Crossfire enabled (I assume this is a setting required for Eyefinity to work?) . Plugged it in and voila, the third monitor works fine. I checked the refresh rate in the Catalyst Control Center and it showed a refresh rate of 60Hz. So I'm not sure if you only use one card, whether it allows for such a high refresh rate or not.