I know this is an old post by now, but maybe someone else is wondering.... Im not sure about single player/story but it definitely works for Terrorist Hunt SINGLE player. It does not work for any multiplayer; VS or TH coop. And you have to reenter the code every time you start playing or change the map. If you die and replay the same map you don't have to do the code again.
For anyone who wants to try it, here is the code for Xbox:
To go into third-person mode while playing a single player campaign, [Press Start/Pause] hold down the right bumper and insert the following code: X, B, X, B, [Press Down] Left Analog Stick, Left Analog Stick, Y, A, Y, A, [Press Down] Right Analog Stick, Right Analog Stick. If done properly, a message will pop up to inform you. If you want to turn off third-person mode, simply insert the code again.
Also you don't have to do this super quick for it to work and the "pop up message" can be hard to see so the best way to see if it works is to unpause the game. And... remember to hold down the right bumper (first step) the whole time you enter the code.
Finally if here's what it looks like (not my vid btw):