The Triple Kill medal is obtained by killing 3 opponents within 4 seconds of each other.
The Triple Kill medal is available in all multiplayer gametypes and has a Bungie difficulty rating of 6 out of 10. It is represented by a steel circle with 3 turquoise stars. When someone gets this medal the announcer will shout "Triple Kill!" to the player that received this medal. In Halo: Combat Evolved, 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.
For a very easy way of getting this achievement and the Steppin' Razor achievement play a game of Swords. It is easy to get a triple kill with an Energy Sword if the player is even somewhat experienced. Another easy way of getting this achievement is to destroy a full Warthog.
An interesting observation is that the difference between the likelihood of a Triple Kill and an Overkill (the next step up) results from the fact that most people will run out of Battle Rifle ammunition after their third consecutive kill.