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.
AMD Customer Care
The information presented in this document is for informational purposes only and may contain technical inaccuracies, omissions and typographical errors. Links to third party sites are for convenience only, and no endorsement is implied.