My advice would be to get a three blade in the range of the mfg book that came with the engine and go fly it. Then, taking how it flew, change to a different pitch or diameter and continue on until you find your perfect combo. The biggest problem with a three blade in your circumstances is due to rpm. The high rpm is what makes a three blade less effective than a two.
Now being that OS recommends the 14x6-8 I would say a 13x8x3 would be a good start.