I am working off dim memories here, but ISTR that Klaus had vibration
problems with an experimental (in the deepest sense of the word)
3-bladed prop and that he recommended 2-blades only for the EZ series
aircraft.

Many pushers now use 3 bladed composite props. The most popular for the
180 HP and up crowd is the Catto Prop, maple laminate core with 25 to 36
layers of fiberglass over the wood with a 1/2" aluminum mounting plate
embedded on one side. He makes them for 100 to 600 HP engines. They
are known to be bullet proof.

The lower power pushers use 2 blades because they can get away with it
and they are much cheaper than 3 bladed fixed pitch props. The higher
the engine HP the more blade you need. A two bladed prop on a 200 HP
engine is just to long for a pusher, so 3 blades are required.

Scott