>! Lock the three chains together in the form of the [Borromean Rings](https://en.wikipedia.org/wiki/Borromean_rings), and hand me the linked set, then walk away so that you can't see which one I unlock. I unlock the one that I have the key for, separating all the rings. Then, leaving them all unlinked, I relock the chain that I unlocked, and return three unlinked chains to you.  
>! It also works in "reverse", where you give me the unlinked chains, and I return them Borromean-linked.