You had three bodies in one multibody part and Shared Topology connected the mesh. Why do you need to Boolean these bodies together?

These are well connected meshes.

There is no problem performing a Boolean on those bodies if that is the last operation. However when you have a Share Topo item in the outline, you can't do a Boolean.  In fact, Share Topo is not required to Share Topology. It is an obsolete function.

18.2 archive attached.