Refering to the nth copy

I’m trying to make a shelf with multiple racks which are divided in different sections.
Is it possible to refer to the nth copy of a component, for example every third copy has a different position than the others?
Sketchup file(probably overcoplicated):
Shelf.skp (2.6 MB)

In the Dynamic Components User Guide read about the Copy attribute …

Not directly a response to your question, but looking at your model, this old thread might be of interest.