*Must be done on Normal or higher difficulty*
For the triple, you must be using the jet pack. Not while you are free falling or have run out of thrust.
During Mission 7, "Exodus", you will go in a elevator that takes you to a open part of the tower. You will see banshees flying around and a couple troops in front of you. One of these troops has a rocket launcher. Swap out your weapon for the rocket launcher with the and hold on to it. You will walk a little to the left and be told to grab a jet pack. To use the jet pack, you press the and the to go in the direction you want. After you jettison across a couple platforms in the Cargo Port, you will come to a bigger platform with enemies on it. While using the jet pack, shoot the rockets at the 3 grunts on the lower of the two platforms for a easy Triple Kill.
This can easily be done during a Bonus Round in Firefight. Large groups of grunts will be dropped down(8-12 in one group). Get ready for the Dropships and sit on top of a higher part of the level(i.e. a rock or rooftop). Once they jump out, start to jettison into the air and shoot a rocket down at them. Refer to "Heat in the Pipe" for more information.