I was reading a manual for that series and it says this:
"Your ATI Radeon HD 5750/5770 graphics card provides multi-display functionality
through its various display connections. The graphics card can output to a
maximum of three displays using the following set of connections:
• Two DVI-I Dual Link connections and one DisplayPort connection."
so that's sounding to me 2x dvi and a displayport at once for 3 monitors is fine.