Generally, yes, the RPMs should be about the same and typically the goal when changing from a 2-blade to a 3-blade would be to maintain the same max RPM. But depending on the prop blade design (i.e., how draggy or efficient it is), and what the diameter of the original 2-blade prop is, it may take more or less than a 1" change and/or some pitch change as well to maintain the same load on the engine. By adding a blade, you're adding about 50% more mass (and drag?) regardless of the diameter of the original 2-blade prop. But reducing 1" in diameter on a 10" prop is a 10% reduction in mass whereas a 1" reduction in diameter on a 20" prop is only a 5% reduction. I don't think there is a one size fits all conversion.