This trophy is missable. You don’t have to kill 3 enemies at once, just 3 over the course of the game. When an enemy investigates a sound or a dead body or fights a wild animal, you can easily grab a stealth kill.
You can throw bottles and shoot arrows at walls to distract them, or even string enemies up in trees to have others investigate the body. If you need to, you can farm the 3 kills via checkpoint restarts.