Is it possible to connect two external monitors to a Retina MacBook Pro without using Thunderbolt displays?
For example, one connected with HDMI and another using Apple's DVI connector and also use the screen on the laptop.
Yes, that should definitely be possible, as both Thunderbolt ports are also mini DisplayPort ports and the other would be connected via HDMI.
See also this answer: https://apple.stackexchange.com/a/54306/14994
Yes. In addition to the HDMI port, you can use either of the Thunderbolt ports as mini DisplayPort ports, just like all Thunderbolt ports.
I can confirm this. I have three monitors connected and running in addition to the retina screen. two via the thunderbolt ports and one hdmi
However this caused my retina display to flicker.