All three of my monitors are detected on my PC but one is still black. One monitor is plugged into the DVI port via an DVI -> HDMI adapter and an HDMI cable, the second monitor is plugged directly into the HDMI port and the third monitor is plugged into the Mini DisplayPort via a Mini DisplayPort -> HDMI ACTIVE adapter (or so Amazon says). I have spent over $150 on adapters in hopes something would work and I am tired of wasting my money on something that doesn't help.
This is the image of my "Devices" tab on my PC showing that all are recognized. Just to possibly help, I am using an AMD HD 7900 series graphics card and everywhere I have looked says it supports up to SIX monitors.