@OP: Jailbreaking isn't that simple. You have to be on a firmware that's already been customized such as the 3.55 firmware. If you already updated to the latest fw, only option would be to downgrade to 3.55 then jailbreak. Or maybe wasting another 200 bucks on a new PS3.
Secondly, after being on a CFW, you won't be able to go online without a spoofer. You'd have to manually search for a spoofer for the latest update out for PS3, which is 4.20 I think. This will allow you to bypass Sony's restriction on going online without the latest updated firmware.
Lastly, you'd have to know how to install FTPs in order to add in modded patches to games you'd like to hack. A lot of people use Comgenies Awesome File Manager which can be accessed and used on the PS3 itself without having to use your computer to file transfer. As Revive stated, you cannot mod MW3 as of yet due to there not being modded patches out for it as of yet. However, you can modify previous CODs such as Black Ops, MW2, WaW, and COD4.
I hope I helped you.