https://wccftech.com/halo-3-mod-third-person-mode/

A new Halo 3 mod introducing a third person mode has been released online.

The mod, developed by parapara, introduces the third person mode in the Halo 3 main campaign mode, complete with a highly detailed 3D model of main character Master Chief. This mod can be used in co-op mode, but all the players need to have the mod installed. More information can be found on Nexus Mods.