Actually, that depends on the resolution of the middle monitor, what games you play, and what settings you want to play them on. we'll need that information to make more accurate recommendations.
A 6750 coould easily support 3 monitors for productivity, even 3 1920x1080 monitors. But you don't want to game @1920x1080 with that.
A 6770 would easily support 3 1280x1024 monitors, and game very well on the middle one.