im guessing you've tried this but ill mention it anyway.
try deleting everything to do with mw3 game data AND save data ,
turn off your ps3.
hold the power button till you hear 2 beeps , this will take you into the recovery menu , then select "Restore File System" once this is done turn your ps3 back off.
then turn on normally , check mw3 disc is clean then reinstall update and start fresh on the game.
Hope this helps