The Hydrodynamic Diffraction analysis can be performed with or without hydrodynamic interaction effects. If you do not include interaction, a separate hydrodynamic database will be generated independently for each structure; in the Pressures and Motions result you will only be able to plot results for one structure at a time.
However, if you do include interaction, the radiation damping and added mass calculated for each structure (and subsequently, RAOs etc) will account for the presence of the other structures in the interacting group. Then, when you plot Pressures and Motions results, you do so for the whole group.
In your case, where multiple structures are joined together in close proximity, I would recommend performing the diffraction analysis with hydrodynamic interaction turned on: click on the Hydrodynamic Diffraction Structure Selection, then click Interacting Structure Group 1, and select all of the structures in the Graphical Window. Then click Apply in the Interacting Structure Group 1 field.
I hope this helps.