You must own a legitimate copy of (Premium Edition with all 5 DLCs is required for most servers).

Create a VU account and link it to your account. Launch and Install :

Run the VU client (it is recommended to run it as an administrator). Follow the in-client prompts to create a soldier profile. : Inside the VU client, go to the Server Browser .

The game must be installed and launched at least once before installing any mods. :

If you are looking for a major Battlefield 3 modification, the is the most prominent project available. How to Download and Install BF3: Reality Mod

The mod files will typically download automatically once you join a server.