"Loot 3 chests" is an optional mission objective in Sequence 6 / Memory 01: Diving For Medicine.
There are a total of 6 chests to be found in the wreck.
Walkthrough[edit]
Go straight forward from the Diving Bell. Dive down to the sea floor and collect the first chest. Now swim over to the right, toward the next part of the wreck to the North. Take care to avoid the jellyfish. Inside the hull of the bow you will find the next chest. After that swim up the mast to collect the first of three Animus Fragments. From there swim to the right-hand side of the hull section to the North. Inside there you will find the third chest of the dive (completing the first optional objective)
