To view achievements for Halo 3, complete the following steps:
1. Tap the Xbox Guide button located in the direct center of the controller. The button will have a bright green light in one of the four corners on the button, depending on what player number you are.
2. Tap the left bumper once. The bumpers are the horizontal shoulder buttons on the controller. The left one is marked "LB".
3. If you have followed the steps correctly, there should be an option on the top of the screen reading "Achievements". To the right of the option is a number. This number can be anything. The number shows your gamerscore amount.
4. If you have played any games of Halo 3, then once you have selected the "Achievements" option, there should be an option. This option will say "Halo 3". To the left of the "Halo 3" word, there should be a customized "3". Select this option.
5. Now, it lists all Halo 3 achievements! Select on any option to view a complete description on how to obtain the corresponding achievement. If you have not played any Halo 3 games, view Step 6.
6. If you haven't played any Halo 3 games on your profile, go to http://www.xbox360achievements.org—REMEMBER: You must have an Internet connection. Once you are at the website, view the left panel. Select "Retail Games". from there, scroll down the alphabetical page and select "Halo 3". By there, it lists all Halo 3 achievements. View step 7 for the most important step..., or use halo.wikia.com
7. Good luck and have fun! :)