3-blade vs 2-blade propellers
I have done some research in forums about 3-blade propellers for multicopter. The main advantage is the smaller radius, which makes it easier to not touch the ground with the propellers. The smaller radius will also let you build smaller multicopters. The disadvantages is that they are more expensive, heavier and slightly less efficient. People say they get less time in the air with 3-blade propellers.
When comparing a 3-blade propellers performance with a 2-blade propeller, a rule of thumb says you should add one inch to the radius and take one inch from the pitch, so a 9×5 3-blade propeller has about the same performance as a 10×4 2-blade propeller.