If I launch a Joomla! 3 site, will I be able to install extensions?
From Joomla! Documentation
Revision as of 22:14, 14 April 2013 by Tom Hutchison
Yes, you will be able to install extensions compatible with Joomla 3.x. The Joomla Extension Directory (JED) will have a compatible icon to know which extensions are Joomla 3.x compatible; however, the best place to check is the developer’s site.