ممكن حد يقولي الإستعلام التالي بتاع أى هاك بالظبط إذا سمحتوا و أكون لكم من الشاكرين

SELECT user.userid, user.username, COUNT(post.postid) AS replycount
FROM post AS post
LEFT JOIN thread AS thread ON (post.threadid = thread.threadid)
LEFT JOIN user AS user ON (post.userid = user.userid)
WHERE thread.forumid=3 AND thread.firstpostid != post.postid
GROUP BY post.userid
ORDER BY replycount DESC
LIMIT 1;