ORIGINAL: bigedmustafa
The only way I've noticed that 3-blade props are less efficient than 2-blades is when you whack them. You get to spend twice as much money replacing, so in that respect anyway, they're less efficient.
The financial efficiency (or inefficiency) is a lot easier to understand than the aerodynamic stuff. I'm cheap (or "efficient") so I'll have to stick with two blades.
My rother replaced a 12x6 two blade with a 10x6 three blade on a trainer and got similar rpm numbers with both props. The two blade did have more static thrust; but, on the trainer, flight performance was pretty similar.