I have 3 monitors. One connected by HDMI, one by DVI, and the last is connected with an active DP to DVI adapter. My computer and the AMD settings know there is a third monitor, but for some reason they won't let all three work at once. I have to disable one monitor if I want to use my third monitor.
I use an AMD Radeon 7800. It's compatible with Eyefinity. It's supposed to support up to 4 monitors.