What if an extension only works on Joomla! 2.5 and not 3.x?
From Joomla! Documentation
Revision as of 22:40, 14 April 2013 by Tom Hutchison
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.