I don't think your rpms should drop. The main point of a three-blade prop (aside from things like ground clearance or scale looks) is to get more thrust from a given rpm. As I understand it, the fact that the tips of the blades don't have to move so fast with a three-blade prop (because the blades are shorter), which makes this possible in principle even though the engine is pushing more total blade surface are through the air. But this is not something I know a lot about.