Facebook, google and twitter login only

You mean to disable standard Joomla registration? That's easy to do:
* In the Joomla User Manager, click "Options" and set "Allow User Registrations" to "No".
* In the SCLogin parameters, configure it so that the Register button and login form are not shown.

That should do it. We haven't done a ton of testing with that scenario, but have been told it works fine.

I hope that helps,
Alex