Are you running Windows? If so right click your desktop and click 'Screen resolution'. Do all the displays show up? You should have 3 displays show up
If i connect my third monitor all the monitors go blank, so i cant even see if it does, but i hovered my mouse over the detect button in screen res as i plugged it in, monitors went blank, I clicked detect, and then unplugged the monitor to see that it did not know that their was a third monitor plugged in.