Can completion of ALL drag & drops be used to trigger a new layer?

I have students exploring three options by going to each one via a drag and drop interaction on a hub slide.  Each option has several slides attached to it.  After the students have explored all three layers, I want a new layer to show (and/or a button to appear to link to a new slide) on the hub slide.  I set up some variables so that when "Drop Correct" is set for each of the three options on the hub slide, this new layer/slide button will show, but it's not working.  I think that when the drag and drop takes the student to other branches, the "drop correct" state does not persist on the hub slide, and when they come back to make another selection, the dropped state is reset (that's my guess).  First of all, is there a way to do this - have the "drop correct" state persist even after students go off to other slides and come back to the hub slide?  Second, is there an easier way to do this?  The idea is to let the students explore the three options (but require them to do all three options) before going on.