Only 2 "native" DVI/HDMI connections normally exist on AMD cards(DVI/HDMI are the same signal). These use a clock signal. They are "shared" between all the Ports. So a passive MDP to HDMI cable DOES work(it just passess the signal thorough). BUT uses one of those 2 clock signals.
The third monitor must ALWAYS use an active display port adapter. This is because display port does not natively use a clock signal and the adapter has to be able to add one to covert the signal to DVI/HDMI as the other two are already used
As I said you have to use an active MDP to DVI, then a simple DVI to HDMI(same signal just different plug) because there are no active MDP to HDMI adapters (there is a coming soon section on AMDs site now, but still no adapter yet) . probably because people running multiple monitors almost always use DVI instead of low end monitors/HDMI which is really for TVs. But simple to convert anyway. So no big deal.
Active mini display port to single link DVI
DVI to HDMI