https://wccftech.com/halo-reach-mod-enables-third-person-view-in-campaign/

A new PC Halo Reach mod uploaded on NexusMods by user AkFumbles enables third-person view in the game's campaign mode.

In order to do so, you'll have to go into your game's installation folder, move into the maps directory and replace the original map files. As always in these cases, it is a good idea to back up those files somewhere else before performing the Halo Reach mod for third-person. Of course, you'll also need to launch the Anticheat disabled version of the game.