Triple Kill is an achievement in Halo 3. It is awarded when the player gets a Triple Kill medal (three consecutive kills within four seconds of each other) in a ranked free-for-all playlist. This achievement gives 5 
to the player.
Following the shutdown of Xbox 360 Halo online services in December 2021, this achievement will no longer be attainable.[1]
Rewards
Unlocking this achievement rewards the player with the following;
- 5
- Commando chest multiplayer armor permutation.
Tactics
- An easy way of getting the achievement is to, during any match with Energy Swords, grab an Energy Sword, get on top of a high structure, and jump and surprise the large battles on the ground floor. Tip: Only lunge when there is roughly a ten-foot distance between you and your targets, or if the reticule turns red.
- If you are playing King of the Hill on The Pit (or Pit Stop), man one of the turrets and wait until others charge the hill.
- A good way to get this achievement on Construct is to get a powerful weapon (such as the Energy Sword) and camp at the top of the central lift. Lots of people will fly up the lift, and another bunch of people come in through the hallways.
- One effective method would be to wait with the Oddball near elevators (such as the main one on Construct) and instantly kill all the players who will try to kill you.
Trivia
It is possible to unlock two achievements at once. If a Triple Kill is accomplished with a Sword, the player will unlock both the Triple Kill and Steppin' Razor achievements, and if a fourth player is killed, the Overkill achievement can also be obtained. On certain maps, other objects like the Mongoose or the Shotgun can be used to get a second achievement.
Sources