User Preference Drag and Drop Submit Trigger

Hello!

I am creating an activity for users to drag and drop 3 out of 12 objects onto three different drop zones(users will rank their top three).  There is no correct or incorrect answer, it is all user preference.

I am having issues setting the trigger for the submit button.  Users should only be able to submit after any 3 of the 12 have been placed in the drop zones.  I'm not sure what the conditions should be.

Any tips or direction would be appreciated!  Thanks!