Selecting a particular object when using 3D Extrude

Hey!

I wonder if it's possible to select a particular object when applying the 3D Extrude effect to an entire group. I had to apply the Extrude effect to a group of objects (all various boxes on the screenshot grouped together) as otherwise the 3D-generated structure isn't kept intact.

I would like to increase the depth of the green box indicated by the red arrow (make it appear like it's higher, protrudes a bit over the gray box)., but due to the grouping I'm not able to adjust the effect parameters of particular objects (when I select a particular object with the direct selection tool, the 3D and Materials menu doesn't show up).

Is there a way to do this?

Thanks