Dragging & targets question

OK here’s the question. Say I have 3 target MCs and 3 draggable buttons (all the same).

all 3 buttons needs to be dragged onto a seperate target mc (doesn’t matter which). The thing is, how would I say/express - ‘if 2 of the same buttons are placed on the smae target mc, do this action to tell the user its wrong!?’

thanks