CONNECT MULTIPLE LAYERS

I have a 3-layered overlap joint, and because the surfaces used to create the shells were extracted from one or the other surface of the solid part, the surfaces/shells have a gap between them.  How do I connect the surfaces to each other to act as a bonded layer?

Here is specifically what I have:  shell layer of steel, then a layer of adhesive (using solid elements) which one surface is in contact with the steel layer, then a gap, then a layer of shells.