I just upgraded an installation from 2.0b4 to 3.0.2. The only problem is that the web interface has no formatting... like CSS isn't loading or something. Any ideas?
Look at the apache.cfg and cgi.cfg and check if all paths are correctly written. Maybe Nagios 3 has put the stylesheet files elsewhere or has the default configuration of those .cfg files which might be different from yours.
Excellent! It was the url_html_path directive in cgi.cnf that was set incorrectly. Thank you!