Three Johnsons glitch

The Three Johnsons Glitch is a simple glitch that can be performed on the Halo 3 Campaign levels, Sierra 117 and Crow's Nest, although slightly difficult on Sierra 117.

Sierra 117
Start at Mission Start. Go to the waterfall and wait for Johnson. When he stops and looks at you, shoot him in the head. Then go kill the enemies that jump out of the first Phantom and get a loading point or checkpoint. Then head back to where the first Johnson. Melee and headshot him whenever necessary until you reach the area where the second Johnson is captured. When you see the second Johnson, headshot him with a Battle Rifle and use a Brute Shot and grenades to get him down. Finally, melee the two Johnsons to the area where the third Johnson is.

Crow's Nest
Start at Mission Start. Melee the first Johnson to the hangar. After you have killed all the enemies, melee him back to the command center and melee the second Johnson into the barracks then the first Johnson. When you get to the area with the third Johnson, headshot the third Johnson after all the marines have entered the Pelican. This will prevent Johnson, and also the Arbiter, from being removed from the level. Now you have three Johnsons. However, the third Johnson and the Arbiter will be removed from the level when you reach the room with the Plasma Cannons, so this will leave you with two Johnsons to melee to the end of the level.

Trivia

 * You can kill any of the Johnsons who respawn with a Battle Rifle anytime to get near infinite Battle Rifle ammo.

How-to Videos
Hold X to Trick - Three Johnsons