Yeah, for gaming on three monitors the HD5770 will be lacking. It will be more than fine for everything else on 3 monitors however. If you are willing to game on just one of the monitors then it's an ok choice. Otherwise you'll want at least the HD5850 or 2 of the HD5770s in crossfire.
jyjjy is absolutely correct in that you will do fine if you only game on one of those monitors and use all three when not gaming. However a second 5770 will not help since it is still memory bandwidth deprived by a good amount.