The rule of thumb when switching from a 2 blade to a three blade is down an inch in diameter up 1 in pitch.
When going from a 2 blade to a 3 blade the RPM will change. You gotta remember that third blade is a lot of load on the engine.
You switch to a 3 blade for a few different reasons, 1 clearence, 2 speed, 3 noise. When I say speed I'm talking slowing on down lines, not faster in level flight.