All good questions.

3-blade propellers offer two distinct advantages over comparable 2-blade propellers:

A) Better braking

B) Better ground clearance

A three-blade prop can be more effective at slowing down a plane. When you're coming in for a landing and you reduce the throttle to idle or near-idle speed, that third blade provides extra wind resistance out on the nose of the aircraft and it can help the plane bleed off more speed while making a landing approach. Three-blade props are also often used to provide better ground clearance on models where the prop just can't fit too far above the ground. My old Tiger Stick .40 ARF had this problem, and using a three-blade prop was a nice solution.

The general rule of thumb is that you want to drop 1" of diameter when adding the extra blade. If your .46 AX turns an 11x7 prop nicely but you just don't have the ground clearance, try a 10x7 3-blade prop instead. If a 14x6 2-blade prop is a good, standard prop for your .91 4-stroke, but you're putting it in a .60-size P-40 Warhawk and you want a 3-blade prop to be more scale looking, just go with a 13x6 3-blade for the same engine loading.