You just need to do the following (don't feel like using tags):
Event: A unit acquires an item
Condition: You need to check if the unit has the items that it takes to combine
Actions: If it does, remove them and add the new one. No else.
That's the idea, and the execution is up to you since there are many fancy, and less so, ways of doing this. That ok, or do you need the details? | 
