If all other parameters (such as pitch , airfoil and blade shape) are unchanged the diameter of the 3-bladed propeller should be 90% of the diameter of the 2-bladed propeller in order to allow the engine to swing both propellers at the same rpm and power.

The reason for this is that the power requirement scales with the fourth root of the number of blades, i.e. for a 2- two 3-blade conversion the scaling factor is (2/3)^(1/4)=0,9036...

In real life the rpms may deviate slightly because the smaller diameter propeller operates at a lower Reynolds number and also the interference between blades is more pronounced with a 3-bladed propeller.