Craig,

The theory on efficiency goes something like this:

As a blade moves through the air, it disturbs it and leaves turbulence behind it. As the next blade comes around, it will be most efficient if it is seeing un-disturbed air. With a 2-blade, the approaching blade is farther back than with a 3-blade. So the 3-blade is operating in more turbulent air than a 2-blade. This is why the 3-blades are more inefficient. Obviously, the faster the prop is spinning, the more noticeable this effect is. That is why you almost NEVER see 3-blades on small sport planes--the performance is just terrible.

Ken