You must be a soldier to earn this, but most of the soldiers primary weapons work. Crits happen randomly, percentage based on server settings. If you find a server with 100% crits, save it, so you can work on this achievement.
Avoid any items that grant a guaranteed crit. Those are usually a mini-crit, not a real crit. Mini-crits won't do enough damage to kill three people. Wait for a real crit.