"Kill three guards from behind corners" is an optional objective in Sequence 2 / Memory 06: The Treasure Fleet
Walkthrough[edit]
Do this at the beginning of the memory, when you need to retrieve your gear
Killing the guards from corners is the best approach anyway (since you are unarmed and direct confrontation might not end so well...)
Get close to a wall's corner to automatically hide behind it
Then, perform stealth assassinations from there when the enemy comes near
If he does not come to you, whistle to grab his enemy's attention.