First:
You must be Level 80. Go to CoD Elite and Sign in.
Second:
Go to your classes and change something, i.e a camo or just a gun (Dont do any of that inspect element stuff).
Third:
It should tell you to send changes to the game but do NOT click it yet. Now in MW3 prestige yourself.
Fourth:
Now, play Multiplayer until you reach Level 4.
Fifth:
Now that you are in Level 4, in CoD Elite click on send changes to game.
Sixth:
In MW3 accept the changes of CoD Elite and BOOM your classes are back!!
Hope you guys like it