RE: 3Blade Prop for OS BGX 3500
We don't have a recommendation for a 3-bladed propeller. It all depends upon what flies the airplane the best. We'd also suggest a 2-blader if you don't have ground-clearance issues. They just work out better, and there are more props to choose from so that you can tune the airplane for good performance.
We normally run an 18 x 6 or 18 x 8 prop on the O.S. BGX-1 3500, so possibly a 16 x 8 3-blader may work...or something similar.
Basically, find what RPM the engine runs best at and then get a propeller that will give you that same RPM and fly the model how you like it.