I am using a ThinkPad X230 in a mounted tray, with its own power button, USB ports, etc. It lacks an HDMI port but it has a DisplayPort opening. I have a DisplayPort-to-HDMI adapter, and I have used that, and an HDMI cable of course, to attach a second external monitor. (The other is connected via VGA.)
In dom0, in the Display settings, the 3rd monitor is detected, but underneath the name, it reads “(Disabled)”. There is no apparent means of enabling it. When I turn it on in the applet, there is no change on either the monitor or on the blacked-out, “(Disabled)” status of the monitor, even though the switch is now in the right position.