I have a 7850 card and went through lots of config to get 3 monitors working...
I eventually set it up with one DVI, and 2 mini DP to DVI adapters.
the adapters actually came from the mac store, they claimed they are "active" but I would reccomend an "active mini DP to DVI adaper"
any other config gives you 2 or 3 desktops..
you will get video on all 3 but you cannot eyefinity all 3 to 1 desktop.