From Zinger Props ...
The conversion for a three and four bladed propeller is a simple tasked if you know what two bladed propeller you use on a given Engine. For example, on a three bladed propeller you would drop the diameter and keep the pitch. On a four bladed, you would drop the diameter and the pitch. For
instance if you had a two bladed 24-10 propeller and you wanted a three bladed you would use a 22-10 and so on.
Anyway, here's the link to the page.
http://www.zingerpropeller.com/_2_blade_.htm