Adam,
Sorry for the delay! For JomSocial 3, there is actually built-in support for JFBConnect. Please go to the JomSocial -> Configuration -> Facebook Connect section and enable the "Enable JFBC" setting (or whatever it's called in the bottom left). For more information on the best way to integrate, see our
JomSocial Facebook integration
page.
Enabling that setting will automatically add the JFBConnect 'Login with Facebook' button to the JomSocial home page.
For the LinkedIn button, the steps in the page you linked to should work work. Please make sure you're looking at the /components/com_community/templates directory, not your /templates directory (for your Joomla template).
Alternatively, if you want to wait about a week, the upcoming JFBConnect v5.1 release will have a very easy way to automatically add JLinked's LinkedIn button to the JomSocial homepage as well as the Google+ support, which the v5.1 release will also have.
I hope that helps explain, but if you have any other questions, just let me know.
Thanks,
Alex