What if an extension only works on Joomla! 2.5 and not 3.x?
From Joomla! Documentation
(Redirected from What if an extension only works on Joomla! 2.5 and not 3.0?)
You need to speak with the extension developer to see if they have plans to release that extension for Joomla 3 in the immediate future. Developers do have to make some changes but how extensive they are depends on the extension.