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.