I know PHP-Nuke, Joomla, and e107 use phpbb2. Is it possible to use any one of these sites and use PHPBB3 forum?

OR

I know the Phpbb3 portal comes with the subsilver theme in the download, but what if I use another template I've downloaded? Will it still work? Or is the portal theme only for the default phpbb3 themes?

phpbb3 is just over 4 months stable. Give it time, things will (eventually) migrate there. Till then, I think you'll be hard pressed to find something.

I think joomla had something in the works a while back, don't know now though.

Check this (http://www.rocketwerx.com/forum/viewforum.php?f=22) out.