when does one start to use them? does it do anything better than 2blade? any advantages? how does one compare the size to a 2blade?

All good questions.

3-blade propellers offer two distinct advantages over comparable 2-blade propellers:

A) Better braking

B) Better ground clearance

A three-blade prop can be more effective at slowing down a plane. When you're coming in for a landing and you reduce the throttle to idle or near-idle speed, that third blade provides extra wind resistance out on the nose of the aircraft and it can help the plane bleed off more speed while making a landing approach. Three-blade props are also often used to provide better ground clearance on models where the prop just can't fit too far above the ground. My old Tiger Stick .40 ARF had this problem, and using a three-blade prop was a nice solution.

The general rule of thumb is that you want to drop 1" of diameter when adding the extra blade. If your .46 AX turns an 11x7 prop nicely but you just don't have the ground clearance, try a 10x7 3-blade prop instead. If a 14x6 2-blade prop is a good, standard prop for your .91 4-stroke, but you're putting it in a .60-size P-40 Warhawk and you want a 3-blade prop to be more scale looking, just go with a 13x6 3-blade for the same engine loading.

awesome thanks...

They are liked AND disliked for the very same reasons. They provide good thrust, but at the expense of lower efficiency, which gives good thrust and good braking. Plus, gives better ground clearance.

A lot of pattern folks use them becaue of the braking caused by drag.

And, they look pretty neat too.

CGr.

something to remember..........the 3 blade props that come on the PTS Raptor is just horrible.....then again I've seen a narrow 3 blade prop on a Hangar 9 Hellcat improve the performance over an APC 2 blade.
You just gotta experiment and see what you like.

I will get both... an APC 14x6 and a 3blade 13x6... which company's one must I get?

I think Master airscrew makes a narrow bladed 3 blade

"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.

Graupner 3 blade props are very good but they are heavy and thus throttle response is slower. If that doesn't matter than the graupners are the best.