If you're doing it more visually and simple then you can have a variable tracking the number of items, and depending on the number when you drop, place it in a specific place. So if numItems is 2 then on drop set item to slot 3. This won't work if you can pull items from the middle though so 2 is empty but 1 and 3 are filled.
You could also use on object dropped, if not overlapping item i.e. there isn't one in the slot, then place it there. That's using the presence/collision of the actual sprite object.
You could also have the slot as an object, with an instance variable which tracks whether an item exists. On dropped over slot, if isEmpty(variable), then place the item. But this doesn't choose next slot and rather relies on the specific slot you are choosing to drop it in.