I'm not going to comment on the 24" prop thing. They work great on 60cc and up size engines.

The three blade props, properly sized, will usually provide a tad bit better vertical performance, but don't brake as well on the down line. The noise level will drop a little, and the rpm numbers will be similar to what you have with a two blade.

To change from a 2 blade to a 3 blade, drop one inch in diameter and add one step up in pitch from what you had with the 2 blade. The size steps don't always work out when transitioning from 2 to a 3, so the pitch part often needs to be tempered a bit. Bear in mind that spinner costs will go up, along with a significant rise in prop cost,