You've got it. The 7990 can do 2 passive/legacy outputs (the DVI and the mDP-to-HDMI that you've got there). To get 3, use an adapter from www.amd.com/dongles and you'll be good to go with 3, since you just need one ACTIVE (and certified) adapter. There are some non-certified active adapters out there, but we see tons of issues with some of them, so it's better to stick with certified ones.
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