From the MSC NASTRAN manual, RBE2 rigidly connecting the same components of several grid points together. From the example TORQUE.dat, the result of displacement vector shows T2 and R3 is completely same.
RBE3 does not add additional stiffness to the structure. Forces and moments applied to reference points are distributed to a set of independent degrees of freedom based on the RBE3 geometry and local wieght factors. so RBE3 can get different displacement or rotation. The example shows the T3 has different value.
My understanding for difference between RBE2 and RBE3 is whether the connection is fully rigid or not. RBE2 is rigid and RBE3 is not.