I've used a lot of 3 blade RC props for scale effect but never felt they were as efficient as a 2 blade. That is opposite my experience in full scale aircraft where a 3 blade would always put you back in your seat at take-off more than a 2 blade in the same aircraft. One thing about the McCauley 3 blade on my old Cessna 182 was that it had a lot more drag than the original 2 blade when you throttled back. Descents of 4500 feet a minute were possible and it was one of the things I always demonstrated as it could bite you if you did not realize your vertical speed. I imagine a 3 blade on an RC aircraft would have more drag at low power as well which would give you steeper approaches.