Tip:
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First you wanna head over to the developer console by clicking the acute or ( ` ) key.
Now that you've opened the developer console, you want to type "sv_cheats 01" without the quotes.
A message should pop up in-game saying "Server cvar 'sv_cheats' changed to 01"
Once you've enabled sv_cheats, you're ready to get into thirdperson mode. Simply say "Thirdperson" without the quotes into console.
To un-enable thirdperson mode, say "Firstperson" without the quotes.