Theoretically, the fewer the number of blades the greater the efficiency. I think it has to do mainly with how much "disturbed" air each blade encounters from the blade that precedes it. It was well known by model builders that the most efficient set-up was a single blade prop with a relatively small counterweight on the other side of the hub. There are many other considerations of course. Propeller ground and/or structure clearance while still being able to utilize existing horsepower is one issue. I believe that under ideal conditions and everything else being equal, the best 2-bladed prop for a particular situation will outperform the best 3-bladed one based strictly on available thrust. I'm going to use a 3-blader for esthetics, ground clearance, and perhaps some other minor benefits.