"must you get"?????

Either one or both. There really isn't a magic prop nor a magic prop manufacturer. Props, even from the same mfg, vary so much that it's impossible to predict which one will work best for you. There's a good reason for this. The models we hang behind those engines vary so much, and even those engines vary so much that it's really impossible to say what prop is going to suit your engine on your model with your CG location and the weight your model turned out.

And BTW, the idea that the difference in efficiency of a 3blade is going to matter doesn't apply either. The difference in efficiency is less going to a 3blade than going to a lower pitch in the same diameter. Lots of people ignore the efficiency effect when testing 2blades. What effect? Why lower pitched props are less efficient IN THE SAME DIAMETER is the effect. And nobody preaches against trying lower pitch props because of less efficiency.

A 14x7 prop is more efficient than a 14x6. Ever hear anyone tell you to NOT try less pitch because it's less efficient? If efficiency was a realistic concern, nobody would be using flat pitch props. Everybody would be using 14x14 2bladers. Nah.... don't worry about the theoretical loss of efficiency with a 3blader. Try them just like you'd try a different pitch 2blader. Different pitch 2 bladers all have different efficiencies too. The less pitch of any prop diameter, the less efficiency. And the blade area, airfoil, blade width, diameter, tip shape, prop mass, and pitch all affect efficiency. And we often discover something like a 14x5 gives better performance than a 14x6 or 14x7. Try EVERYTHING that would possibly work for you. You'll learn a lot more about flying models that way. You'll also learn that some of the oft-repeated advice doesn't really apply.