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 the it is represented by three white stars on forest green circle rimmed with turqoise 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.