The following comments assume default element dof settings.
In a RBE2, grid 1 is the independent and grids 2-n are dependent. The relative position of grids connected by the element does not change during the analysis.
In a RBE3, grid 1 is the dependent and grids 2-n are independent. Grids 2-n are free to mode relative to each other. The displacement of grid 1 is the weighted average of the displacements of grids 2-n.
RBE2 is used to represent a rigid connection. RBE3 is used for load distribution or a flexible connection.
See R-Type Elements in the Element Library Reference in the online help.
To learn more about RBE2 vs. RBE3 elements take a look to my blog in the following address: