There are three manufacturers of 3 bladed props that are readily available around here. Master Airscrew, APC, and Graupner.

One of them advises you take the numbers from the 2blade that's working for you and reduce the diameter by 1" and increase the pitch by 1.

The 15x6 2blade I'm running on my 91FX would suggest I use a 14x7 3blade. I have run both in comparison testing in a single day, alternating runs. After about 8 flights, I really felt the two were very close to equal in performance on a H9 Thunderbolt. I could only find the Master Airscrew 3blade. The Graupner was special order. APC doesn't make a 14x7.

My Skybolt recently had a test day that was similar. The 61FX on it likes a 13x6 2blade. APC doesn't make a 3blade that comes close. The Master Airscrews available were a 12x6 and a 12x8 3blade. I found the 12x6. It was just slightly less effective than the 13x6 2blade. I've got a 12x8 on order. I've also got a Graupner 12.5x7 3blade on order.