There are few glitches in Halo 3, but in those that do exist, many of them are triggered by the Trip Mine. Note- Almost all of these Glitches are to be performed in Forge. However, if you save you map with the following instructions for the Basic Mine Glitch, you can still use the glitches in custom games.
Basic Mine Glitch
Overload Elephant/Guardians Glitch
This is performed on Sandtrap