Connect two iMacs and an LED Cinema Display to MacBook Pro
The Retina MacBook Pro (13 and 15) both allow for two external monitors using either both Thunderbolt Ports (and correct adapters) or two external monitors (one using HDMI to correct adapter and the other Thunderbolt to adapter).

In both cases, three displays work - the retina display and two external monitors. To add a third display, you'll need a USB display card (there are many and some are cheaper than $30 with performance to match the price) or a software solution to drive the iMac over VNC or similar networked protocol.

Out of the box, the Retina MacBook Pro are not designed to run three external displays so you'll need something extra to make things work.


http://support.apple.com/kb/HT5219#dispnum


See note 4 in the above article for RMBP details.