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.
Two ATI 5770's are about equal to an ATI 5850 (both price and performance wise). It would be a better idea over all just to install a single ATI 5850 (unless you already have 2x 5770's). With today's games, an ATI 5850 will play anything very well @ 1920x1080 resolution.
With a good motherboard like your P55A-UD6 I imagine you've got a decent i5/i7 CPU (hopefully i5-750 or i7-860).
As hunter315 said, Crossfire should be an upgrade path for later on. If you start with Crossfire in your build originally, you sort of start at a disadvantage. Put an ATI 5850 in there now, and you can always add a second one later to improve.
hi thank for all bro, i had already have 2 x Asus 5770, but i cannot connect into eyeinfinity, thats y i want to add another pcs 5750 (got eye infinity). This Asus 5770 got 3 x connection, 1) VGA (connect to 32" LCD TV monitor) 2) HDMI (connect to 32" LED), 3) DVI (connect to 22"). yes my set up I5 750, power supply 850W.
You don't need 3 video cards to do ATI's new Eyefinity. And to be honest, for Eyefinity to work properly you really need 3 identical monitors. Having two at 32" size, and one at 22" is going to play all kinds of games with your eyes.
The point of Eyefinity is to play one video game, with the image being spread wide across three monitors so you get a broader visual experience.