From Halopedia, the Halo wiki
The Triple Kill medal is awarded for killing three opponents within four seconds of each other in standard Multiplayer games (and Halo: Spartan Assault) and within less time in campaign and Firefight. In Halo 4 multiplayer and Spartan Ops, the kills need only be within 4.5 seconds of each other.
Originally, the medal was represented by 3 turquoise stars on a silver circle. In Halo 4 it is represented by three white stars on a forest green circle rimmed with turquoise and grey. The medal is monochrome and imposed on a shield in Halo: Spartan Assault and Halo: Spartan Strike. When someone gets this medal, the announcer will shout "Triple Kill!" to the player that received this medal. In Halo: CE, because there are no medals, if a player kills three opponents within four seconds of each other, they will still hear the announcer, and see the words "Triple Kill!" in the upper right corner of the screen.
The following achievements are related to the Triple Kill medal.