As I understand it, the efficiency is improved as the number of blades is reduced because the trailing blade is moving through through dirtier air (more turbulant, altered angles of attack relative to the airstream, etc.) due to the prop blade ahead of it. The degree of inefficiency is relative to RPM as well...the higher the RPM, the more pronounced the inefficiency. In extreme, some of the really high RPM engines use single blade props (CL guys use them in some events).

Full scale planes really don't have high enough RPM to warrant the need for a single blade prop. In some cases, such as in warplanes, the engine is so strong they need multiple blades for prop clearance despite the reduction in efficiency (the Corsair is an example of how an engine choice drove the design properties of the airframe).