Nope, your LHS guy was probably jumping on the 'alpha trainer' boat, seeign that it uses a three bladed prop. This particular prop was designed for teh beginners, and does in fact have three blades.

In reality, three blads are less efficeint than two blades. The speed in which our model engines run put teh following blade in the wake (turbulence) of teh previous blade's path. So, it never really get 'clean air' to bite. Therefore less effecient

Most (if not all) three bladed props are the same pitch as a two bladed prop. And true, the rule is to generally remove 1" from the diameter, and usually 1 from the pitch when coverting frome a 2 to a 3 bladed prop.

And, as stated, the three blade props allow for more ground clearance.

The question you should be asking is about teh pitch of teh prop..