RE: Difference in performace with 3 blade prop?
If you like it for scale looks..... thats no problem
But prop wise, that 12x7 is a bit too much prop for the .55AX. Read around the forums a bit.
My experience with the .55 engine is that it runs its very best with the stock muffler (and most aftermarket mufflers) propped to turn appx 13,000-14,000 rpm ground peak. An 11x5, 11x6, 12x5 are good places to start. A 3-blade is fair game too - just keep it in that same peak rpm band. Back off on the needle about 300-400 rpm from that peak for flight, and you should be good to go.
If ground clearance is an issue, consider going with the 11" props. You will get better engine performance, and no doubt the plane will perform a bit better than it does with the 3-blade.
The engine does run well with larger props .... so anything down to about 12,000 ground peak rpm is ok. An 11x7 or 12x6 is worth trying. But try not to load more prop on it beyond that.