Not sure about the iMac, but with a 2013 MacPro, I cannot directly drive 3 displays, because one of these is a TV set via HDMI. I'm not sure if it would work with all TB/MiniDP connections. However, I have a CalDigit dock. If I connect the HDMI from the dock, then I can get three desktops. In addition, with Premiere and FCPX, I can set the video output to go to the third display (TV set). This is real video, not just the viewer moved onto that screen.