The "myth" about the inefficiency of 3-bladed props has probably been perpetuated by the Evolution TPS engine setup that comes with many Hangar 9 RTF trainers. It's a good engine, but it comes with a 10x4 3-blade prop that is specifically designed for slower flying. Folks switch them out for an 11x6 or 11x7 2-blade propeller and suddenly have 50% more power, so they naturally assume that 3-bladed props are generally no good. It's just the pitch, not the number of blades, that is the issue.

You should be able to try a Master Airscrew 10x7 3-blade prop with a 3" 3-blade spinner. A .55 AX would turn a 11x7/11x8 3-blade prop, and a .70 4-stroke would let you turn a 12x6 or 12x8 3-blade prop. Dubro is the only manufacturer of spinners that commonly has 3-blade spinners available. I would assume that Hangar 9 makes them in a variety of sizes as well, but I don't usually see them on the shelf at my local Hobbytown USA.

Good luck and good shopping!