I now have this query:

"SELECT * FROM mlf2_entries, mlf2_categories, mlf2_userdata
WHERE (mlf2_entries.category = mlf2_categories.id) AND (mlf2_userdata.user_id = mlf2_entries.user_id)
GROUP BY tid ORDER BY last_reply
DESC LIMIT 0, 5"

That I use to render the following:

Thread title
by username 2 minutes ago in Category

However, I still get the username of original poster. I would greatly appreciate tips on how to show the person who last responded instead. Is it possible or something I should forget?