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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 Extension Directory (JED) will have a Joomla 3.x compatible icon to know which extensions are Joomla 3.x compatible; however, the best place to check is the developer’s site.