Per the AMD site: How many non-DisplayPort monitors can I use with an AMD Eyefinity technology-enabled graphics card?
You can connect up to two non-DisplayPort monitors at one time to an AMD Eyefinity technology-enabled graphics card using non-DisplayPort connections or passive DisplayPort dongles. To enable and drive 3 or more non-DisplayPort monitors at one time, the additional non-DisplayPort monitors must be connected with an active DisplayPort dongle.
No you can use the first 2 display in any combination using the 2 DVI and 1 HDMI connectors. The 3rd would have to use Displayport using the Active Displayport to DVI adapter.
ACTUALLY since your card only has mini display ports you would need to get a mini display port adaptar to regular displayport adapter, if it didn't come with one. And then plug the active adapter into that.