The biggest factor in propeller efficiency is diameter - the larger the diameter the higher the efficiency, the higher the thrust. Everything else being equal, a 3 blade prop is less efficient than a 2 blade prop.
You need enough blade area to absorb the power - more power needs more blade area - get that from increased blade chord, increased diameter or extra blades. Others have mentioned most of the considerations. A 3 blade prop is also smoother.
I guess that you can get details of the above with a good googling session.
i.e. I disagree with jarjar. This question needs more than just simple actuator disc theory.
PS - I have seen a pic of a single blade prop on one aeroplane.