To help accurately:
This will reset your campaign progress and controller settings, but not your multiplayer rank (that is stored on Steam Cloud). cod mw3 offline fix
The "offline" issue in Call of Duty: Modern Warfare III (MW3) typically refers to one of two distinct problems: players being unable to play the game without an internet connection, or a bug where players and their friends appear "offline" even when connected. Addressing these requires different fixes ranging from account settings to network adjustments. Fixing "Offline" Game Access (No Internet) To help accurately: This will reset your campaign
: Start the game and let it attempt to connect to the servers. As it is connecting, either cancel the process or disconnect your console from the internet (unplug ethernet or turn off Wi-Fi). Fixing "Offline" Game Access (No Internet) : Start
: Disc owners may face a "paperweight" issue where the game still demands an update before allowing offline play. You must download the initial day-one patches (often 100GB+) to enable local functionality. Informative Review: Offline Experience Call Of Duty Modern Warfare III - Offline Review
Ensure the "Campaign" and "Multiplayer" packs are fully installed.