Did you have a version with the third item set up? It sounds like it should work. I could take a look and check what you did if you don't mind uploading that version.
The tricky part might be in event 7. Add a for each (family) items, to add all 3 (or more) items to the colliding items list.
However (IMO), drag and drop isn't the greatest system for 3+ item combinations. With drag and drop, mechanically a user is interacting with 2 things, the dragged item and the target.
I think crafting slots would be more suitable if you want bigger combinations, or click /select to add to a list, then combine/cancel. It gets more complicated when having more than 2 items.
Edit: An alternative option is to have a the third item combine with the result of the first two, which could keep things much simpler, but may or may not work depending on your design goals.