I was able to do this with my 2012 15″ rMBP. I just upgraded to the 2015 15″ rMBP and I didn't expect a lesser experience; however, I have not been able to connect the 3rd external monitor successfully yet. I am using 3 1920x1080 monitors connected Thunderbolt → DVI x2 and HDMI → DVI x1.
I just achieved a 4 monitor setup (rMBP display included). Here is my setup:
- MacBook Pro (Retina, 15-inch, Mid 2015)
- AMD Radeon R9 M370X 2048 MB
- OS X 10.10.3
- TB1 port - TB to DVI adapter
- TB2 port - Mini DisplayPort to DisplayPort cable
- HDMI port - HDMI to DVI adapter
The average CPU temp has been between 50-70 degrees while doing normal processing with over 10 applications open. When I start compiling code, I have seen it hit 100 degrees, but once the fans RPM increase, it returns to the 50-70 range.