Create a schema "MySchema"
Create 3 Complex properties "A", "B" and "C".
Create for the 3 Complex inner metadata identified with "content" and type Blob
Create an Event Handler and add "before document modification" trigger
Look in the "Current document properties have changed" property choice:
Notice the output:
where it should ideally read:
The request is to be able to identify which Complex you are taking the property of.