Customisable drag and drop activity with no correct answer

Hi community!

I want to create a drag and drop activity in Articulate Storyline.

There are 6 draggable items, and 3 drop targets. Any of the draggable items must be able to be dropped into any of the 3 drop targets. When all 3 drop targets have a draggable item in them, a feedback layer must appear. It does not matter which draggable item goes where; the point of the activity is to for learners to choose any 3 of the draggable items, and drop them onto the drop targets (1 draggable item per drop target).

I'm really struggling. I have tried setting up variables for the draggable items and for the drop targets, and setting a number of triggers up to try and achieve this but I've been unsuccessful.

If anyone has any suggestions, I would be very appreciative. 

Thank you,

Caitlin