From Master Airscrew ;

To convert form 2 blades to 3 blades you want to decrease the total blade area and
increase the angle of attack (or pitch) to overcome the increased drag of the third blade.
The general rule is to DECREASE propeller diameter by 1-2”, and INCREASE by 1-2”
the propeller pitch. It is all right to keep the same pitch when going from 2 blade to 3,
however you will not want to decrease pitch.

http://masterairscrew.com/images/selecting_a_prop.pdf

Works very well for me.