So i recently had a problem with MW3 every time i clicked multiplayer it said the disc was un-readable. I tested my friends copy of the game on my xbox same problem happened again and again. So i found out it is most likely a problem with your title update. What you need to do is go to system settings > memory > HDD or Memory unit wherever you save the updates and you want to delete the title update and clear the system cache. Next i recomend that you have a flash drive and another account (account does not have to be gold.). First plug the flash drive into your console next sign into another profile when you have done that play MW3 it will ask to install an update next install the update to the flash drive then the multiplayer should work on all accounts. And there your done! :D this is how i got mine to work and same with some of my friends who had this problem.
hope this helps.