Flats-
3 blade- less vibration, less slip while coming onto plane, generaly more tail lift, Less top end speed.
Essential on a flat the 3 blade will tame down the boat, more for a cruiser.
I ran a 3 blade on my old Hondo until I ran a big fat 2 blade that gave it almost 10 mph on the top end. never ran the 3 blade again.
Hydro's-
Lake hydro's tend to like a 3 blade for kicking the tail. I don't know how it effect's the top end.