The quickest way to level up on mw3 is to go on team tac or tdm or whatever you game mode is and put on the support streak and reacon drown this will put you in a helicopter there you fly around the map tagging people this means that on TDM you get 100 xp a tag and 20 xp an asist so this i think is an effective way to level up
Feel free to thank and comment or whatever btw this is y opinion