If I launch a Joomla! 3 site, will I be able to install extensions?
From Joomla! Documentation
Yes, you will be able to install extensions compatible with Joomla! 3.x. The Joomla! Extensions 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.