Roughly speaking, the 3-blade prop should either be 1 inch less in diameter OR 1 inch less in pitch to be equivalent to a 2-blade. Bear in mind that a 3-blade prop will always put more 'load' on the engine than a 2-blade....simply because there's more mass. In the real world though, that doesn't necessarily mean anything. Sometimes a 3-blad will pull better than an equivalent 2-blade, sometimes not. There are so many variables, such as the engine itself, the airframe, etc. etc. etc. Very generally speaking, the more 'torquey' your engine is the better it will respond to a 3-blade and other higher-load props. If your engine is the revver type you'll want to stick with a 2-blade .
As an example, I've always had less thrust with a 3-blade on my 1/2A engines...but my OS 4-stroker (a .91) loves a 3-blade and really pulls with it. In the final analysis, the only real reason to use anything aside from a 2-blade for a flying prop is if you have ground-clearance issues. 3- and 4-blade props definitely look more scale, but that look disappears when your engine is actually running
