Yes you could put 3 graphics cards in there, but im not sure what the configuration you listed is attempting to accomplish. It is a 16/8/4 setup, so which ever card goes in the 4x slot will take a slight hit.
Why do you need a 5750 to go with 2 5770s in CF? The two 5770s in CF could support 3 monitors, are you planning on running more than that? Also, planning on getting 2 5770s from the start seems pointless, just start with something more powerful like a 5850 or 5870, crossfire is supposed to be an upgrade option not a setup you start with.