There are other CMSs besides Joomla! that provide an effective phpBB3 bridge (such as my upcoming CMS). Search it on Google, and you will find many results. Still, I believe that Joomla! provides a fairly useful bridge to phpBB3.
What version of Joomla are you using... If you're using 1.5 then try jfusion's forum bridge. It's for phpbb3. There isn't a bridge that i know of for 1.0 & phpbb3, that i've seen, but that's not to say one isn't out there.