Close Origin.
Download the .torrent file.
Run it under your favorite torrent program.
Once downloaded, unzip file and upload the folder to: C:\Program Files (x86)\Origin Games\Battlefield 3 Beta Trial\
Open Origin.
Enter Key to Origin.
Once authenticated, it will start "installing" BF3 Beta.
It will find the files on the folder and if there are no new updates, the game will be ready to play.
If there are updates, let it update, then start when ready.
If there are issues with the install, remove the folder from the Program files folder and restart origin and load the game through Origin download.