Find unique common element from 3 arrays

Question

Original Problem:
I have 3 boxes each containing 200 coins, given that there is only one person who has made calls from all of the three boxes and thus the

Answer 1

Some improvement in Pelkonen's answer:
From converted problem in OP:

"Given that there is one and only one common element in these 3 arrays."

We need to sort only 2 arrays and find common element.