In general 3 bladed props are less efficient than 3 bladed props. How much less is a matter of debate. Since many of our planes are overpowered, this slight loss of efficiency is often meaningless. When you switch from a 2 blade to a 3 blade, the rule of thumb is to reduce the diameter by 1 inch. The only uses I can see for them is to provide more ground clearance or cosmetics (some planes may look more "scale" with a 3 blade prop).