Most of us don't use three bladed props. They tend to be less efficient. In your case however, they might be a good answer. Running the numbers on Thrusthp, assuming the OS 55 AX will turn a 12x6 prop in the 13000 rpm range (typical in my experience), that will produce a static thrust of aproximately 9.92 lbs. Using the three bladed 11x6, for the same power rating, you should get about 12, 750 rpm (much lower tip speed so less noise), and just about 10 lbs of thrust. In both cases, top speed is about 72 mph (that's not real world speed, as its dependent on air frame drag).

Bottom line, the three bladed 11x6 should be comparable to the two bladed 12x6 with the 55AX, so go for it.

Brad