Right click on the game's title.
There's the drop down menu for automatic updates, tell it not to keep the game up to date.
Here's the problem with that:
If you have MW3 currently installed with auto-updates on, it's already patched to the newest version.
If you don't and just install it now, it'll install the latest version.
The trainer was retired back in august.
You're going to need to find a way to downgrade your game.
* Updated game trainers and cheats daily
* Get notified when new cheats are added
* Request which games get new trainers
* Priority support with any problem