Eric if you are wanting to go to a smaller diameter than the 15" that you are currently using (if I read correctly) to say a 14" 3 blade prop then you are going to need a significant increase in pitch to get the rpm's down. The usual theory for moving from 2 blades to 3 blades is to reduce 1" in diameter and increase 1 to 2" in pitch to maintain a similar rpm. You are already significantly undersized on your prop diameter already so you will need a big increase in pitch to get the prop speed down. A large jump in prop pitch will greatly increase your flight speed. I would think a 14x12 to 14x16 may be needed to get the rpms down to the 7k to 8k range. The speed created from that much pitch may make things interesting