Ideally you would want a slightly larger pitch on your 3-blade prop (as I believe the general rule of thumb for converting 2-blade to 3-blade is decrease the diameter, increase the pitch which would make a 17x14 or similar).

Saying this however an 18x10 could work with the 120SP. Alternatively we have also used the following 3-bladed prop size with good results - 15.5x12.

I hope this helps, however if you have any other queries, please do not hesitate to contact us.

Best regards
Laurenne