Everything piper said is correct but I can think of two other things:

1) A 3-Blade prop is quieter
2) A 3-blade prop gives a better braking effect


There have been some very long and good debates here on RCU over the years as to which one is best, 2-blades versus 3-blades. Just do a search, grab your favorite beverage and start reading.

As Piper mentioned it's also a matter of preference and in the case of competitive flying a matter of need. In competition, having a plane that is quiet does effect your overall score so 3-bladed props and specialized exhaust systems, also known as canisters, are the norm.

Quiet planes helps all of us retain our fields as noise is probably the most common cause of fields being shutdown. Another thing is the bigger the diameter of the 3-blade prop the less efficency is lost which means there is more time for the air to "recover" (meaning less turbulent air) between the blades