Are you getting a for a certain file name, or is the game failing to launch entirely? Managing Content in Call of Duty Games - Activision Support
If you are playing on a community client like Plutonium, they provide their own repair tools or specific file-checking scripts that are much safer than third-party "hot" download links found in YouTube descriptions. cod mw3 all zone files download ff files hot extra quality
Before you hit that download button, you need to understand what you are installing. In stock MW3, game data is packaged into .ff (FastFile) and .zone files. Are you getting a for a certain file