What’s next after Joomla 3.0?
From Joomla! Documentation
A quick summary, if you are using any version of Joomla! except Joomla 3.9.20, you should consider updating or migrating ASAP! There will no longer be a Long Term Support Version with a .5 designation. The latest version of Joomla is the Long Term Support Version. See Joomla! CMS versions for a listing of current and future releases.
Joomla! 1.5 has not been supported for since the end of 2012 and Joomla! 2.5 is EOS (end of support) since 31 Dec 2014. Simply, you should be using Joomla! 3.9.20.