im not sure about the pitts type mufflers, but i know you can probably cut those 3 blade props down to the diameter you need. just get the proper pitch you need, and try to get as close to the diameter you need as you can.
now that i think of it, i believe grish makes small 3 blade props.. i know i have a couple i bought for an .049.
www.grishproducts.com
remember when switching to 3 blade props, use 1" in diameter less than you would with a 2 blade.