RBE3 Element ? HOw to use this with solid elements

HI all!!

I am trying to couple 3d solid tetra elements using rbe3 element and everytime it is showing error with dof saying dpendent dofs cant be constrained...but when i use this rbe3 element with shell elements it worls w\o showing any error kindly help me with how to use this rbe3 element .

and if possible what is the difference between rbe3 and rbe 2 element.