A 3 blade prop does not necessarily produce more thrust than a 2 blade prop. I'm always puzzled that people assume this is true. If your engine is turning a different RPM with one prop than with another, then it is unlikely they both provide the same load to the engine. Why would you assume that one produces more thrust when the load is not equal? Kind of like assuming your car is faster in 2nd gear than in 4th gear because in 2nd gear the engine turns a higher RPM.
You would need to test different props under different flight speeds and conditions to determine which works best on a particular airframe.
Three blade props are usually less efficient due to the proximity of the blades to each other as they travel one behind the other. The turbulence created by one blade affects the air flow over the next blade.