Where did you download mw3 from? If you downloaded a shitty repack delete everything and follow this guide to get the game from Steam. If you already downloaded it from Steam, try verifying integrity of files.
@WeeDStaRRR /moved
All weapons can be unlocked by typing the command
unlockall
into the ingame console (you can open the console with the tilde key).
Camos -> https://forum.plutonium.pw/topic/3810/release-mp-diamond-camo-unlocker-10-cac-slots
@vetxzyto download and run this https://github.com/ChxseH/Redist-Installer/releases/download/latest/Redist-Installer.bat. If you have further issues create a new topic, this one is 2 years old.