Open the console with `~`
Enable cheats if needed: `sv_cheats 1`
Enter `thirdperson`
To return to first person, enter `firstperson`
If `thirdperson` does not work, bind a key to toggle it: `bind p “thirdperson”`
For a custom camera offset, use a third-party addon or server-side script
In some gamemodes, third-person may be disabled by the server or admin settings