With the Invisionfree converter, since there's no phpBB3 conversion, you need to install phpBB2, upload the SQL, then upgrade to phpBB3 to finish the conversion (are the instructions unclear? let me know so I can fix them)
Need a conversion from another forum, but they won't give you the database? Try a crawler converter
. If your converter isn't listed, feel free to post in that thread to ask for one.