Checkpoint64 has a preset for Call of Duty: Modern Warfare III: it already knows where the config folder is, backs it up automatically and keeps every version. Everything this site has on Call of Duty: Modern Warfare III:
Call of Duty: Modern Warfare III
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Config file location
Windows:
%USERPROFILE%\Documents\Call of Duty MWIII\players -
Backup guide
Competitive games keep your progress on their servers — what's on your disk is the config. Checkpoint64 versions the settings folder for 67 games and 16 dedicated servers, so a reinstall or a file verification can't cost you your binds. Free download.