mid-2014 rMBP multiple monitors I have a mid-2014 MBP retina with the Intel Iris Pro 1536 MB GPU running Yosemite. 

Current setup:


1x 27" Thunderbolt Display
1x 27" Cinema Display


I went to connect another non-Apple monitor to my MBP via HDMI or also the Thunderbolt port on the back of my Thunderbolt display however the 3rd external monitor does not detect a single. Searching around I find out that there is a limit to the number of displays a MBP can run although I see some people are able to get 3 external monitors to work. Tried shutting my MBP to enable clamshell mode hoping that it would allow me to have my 3rd monitor to work since it would turn the MBP display off, no luck.

Is there any way I can get this 3rd monitor to work via HDMI or a mini DisplayPort to DisplayPort adapter?

I would be willing to allow my 3rd monitor use a usb video card knowing the frame rate will be far less (I'm not gonna run video or games on it). Would like to know what would be a good adapter that would work.
You can run only 2 external monitors. If you want you can also run the MacBook Pro monitor, achieving in this way the 3 monitors configuration.

The only way to run 3 external monitors is to use an external graphic card to connect the third monitor. 

This answer comes from personal experience and Apple technical details. I've the same model and I've tested all the possible combinations, also trying passive and active DVI connections.