I don't have Battlefield 3 for PC , but I hacked in many different games .
You will need an injector , which you can find loads of them if you use the search button.
Than you will need the process id of Battlefield 3 .You can find it by opening Task Manager by pressing Ctrl + Alt + Delete at the same time .
Add the mass.dll to the injector , enter the Battlefield 3 process and just manually or automatically inject.
If you want some detailed tutorials on how to inject , I'm sure that if you use the search buttons you will see millions of tutorials in different sections.
I believe that Battlefield 3 process is BF3.exe