Halo 3 - Triple Kill Achievement

Kill 3 enemies within 4 seconds of one another in a ranked free for all playlist (Online).

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Gamemode must be Lone Wolves

This is easiest done on the gametypes: King of the Hill or Oddball, since everybody is going for one area. Try getting yourself a rocket launcher or simply throw grenades at the area. This is easiest done at lower rank games. With a little luck, you should have no problem getting this.