alterMW3

1. Extract the file to the Game Dir
2. Select altermw3
3. let it to update
4. play the game
this is the pre alpha release
it may work or wont work
Source
AlterMw3

This will bypass the game registrations and will make you play modern warfare 3 free of cost online. you can use the pirated ones released here on Kat.ph.
but now this software is in pre-alpha test
so there may be some network drops
so play the pre alpha
or wait for the full release
Have fun