Did you originally have phpBB2? If you did, upgrading to 3 doesn't delete your database.
So it's possible to install 2 again and point it to your old database.
If you have only installed 3, currently - afaik - there is no known method to downgrade.
Kepp your eyes on the converters forums, as that's where it's most likely to show up.
I would downgrade from phpbb3 to phpbb2 but I would'nt lose users, topics and posts.
Of course I have my old databse from phpbb2 but I've updated my database to phpbb3 a few months ago so I can't use old database, I need downgrade my database.