Copy relationships from one entity to another

Hi,

I am trying to accomplish something, and there doesn't seem to be an answer anywhere.  Any help is appreciated.

I have 3 entities.  A is related to C with N:N and B is related to C with 1:N.  A is related to B with 1:N.

What I am trying to accomplish is that there are the related entries of C in a subgrid in A.

B would have selected A, in the lookup.

I need to have an option, such as a workflow, that when it is run, all the entries in C, are copied over to the related B.

Thanks guys!