I agree with Dennis and FH, the fewer blades, the more efficient, and the better the power output.

3 bladed props do help with sound as someone mentioned due to lower prop tip speeds. When the tip gets near (if I recall correctly) .6 mach, it starts making a LOT of noise, so going to a smaller diameter, either with higher pitch or more blades, or both, helps. This is why you see them on IMAC planes, they now get scored on noise in competition.

3 bladed props also help with braking in down lines. The more prop area you have, the more braking effect.

This also accounts for the Alpha's prop. The Alpha's prop is a very special case. Next time you see one, note that the blade shape looks nothing like other props you see. They are closer to oars than propellers. The Alpha's prop is specifially set up to PREVENT high speed flight, and reduce the thrust of the engine a bit while still allowing enough power for a decent rate of climb. A more "normal" 3-bladed prop will do some of the same things, but not to the degree that the Alpha's prop does.

Btw, if you want proof, just take the Alpha's 3-blader off, and put on a 10x7APC, and hold on. (A better choice for training would be an 11x5. The 10x7 makes the Alpha go like a rocket)