How do I change Field of View in Call of Duty Modern Warfare 3 Multiplayer?
On both consoles and the PC, the FOV is locked and can't be changed via any official/supported means.
For the PC, I've seen some 3rd party "FOV changer" tools on less reputable sites, but I'm not going to link any of them here. They appear to hook into the game while it's running, and there's a strong possibility that it would be detected by the anti-cheat systems in the game.
Remember that if you are caught by Valve Anti-Cheat (which Modern Warfare 3 uses), the penalty is being banned from multiplayer permanently. The way VAC works, you might not be banned immediately, so even if you or your friends have used a tool in the past and thought you didn't get "caught" you might still be banned in the future without warning.