Halo 3 cheats page 12

[partyjock]

Easy "Marathon Man" achievement

Access the terminals and wait until the red text comes up and it redirects to a different page. It does not matter what difficulty you play on or in what order you access the terminals.
Chuck Lewis
Crouching

To crouch, click and hold the Left Analog-stick. When crouched you will not appear on motion-sensors. This is helpful in multi-player mode when you do not want the other team to find out where you are.
skalagix
Illegal service tags

If you go to "Appearance" and try to change your service tag to I17 (Master Chief's number is 117) the game will not allow it. It says that the tag is reserved for use by the UNSC.
fhwvm Al Cristovao
If you try to use "A55" as your service tag you will be told that it cannot be used under UNCS regulations and rules.
langerdebbie
Getting in the same ranked Lone Wolves game

Go to your Dashboard. Change your language to the second option from the bottom. Start Halo 3. Invite four other friends that are about the same rank as you. Also have them change their language to the same one as you. Get everyone in the same party before you want to get them in the same party. Everyone must search at the same time. Everyone then must make a game in Lone Wolves. Before you go to start the countdown, press X to change the search options to the bottom one, which is to search for the same language. If done correctly, chances are you will be in the same ranked Lone Wolves game as your friends. You can now get achievements or level someone up.
Shadow King X
Be the Oracle

Press Up at the Forge lobby and you will turn into the Oracle. You can pick up items by pressing A when you see a green hand.
ryan robberts
At ease position

Note: The following can only be done in a local match or replay. To do the at ease position you must hold LB + RB for at least three seconds and press Left. If done correctly, your weapon will not be visible on your screen. When you shoot you will raise your weapon again.
lander brandt
Stay zoomed in

If you are zoomed in as a Spartan or Elite and turn into the Monitor, press Up to turn back into your character. You will still be zoomed.
lander brandt
Smarter respawning

When playing in Co-op under the Legendary difficulty setting, it is best to have one player keep their distance from the enemy at all times. By doing this, if the rest of the players die they will respawn around the player that is farther back, rather than having to restart the checkpoint.
brandon mizera
Complete Legendary mode easily

If you are alone and need to complete Legendary mode, turn on an idle second player to stay behind. If you die, you will respawn instead of restarting.