In order to password a BF3 server you need to create a unranked profile and apply it to your server, to do this simply:-

1. Go to Clanforge -> Profiles  and click New

2. Select the Unranked template profile

3. Save the profile

4. Customise it, adding your server password in the Basic tab.

5. Go to Clanforge -> Servers, select your server (checkbox next to it) and click Edit

6. Select the new profile you created above.

7. Save and restart your server.