[kh99]

Maybe something like this:

"SELECT attachmentid FROM " . TABLE_PREFIX . "attachment AS attachment
       LEFT JOIN " . TABLE_PREFIX . "post AS post ON ( post.postid = attachment.postid ) 
       LEFT JOIN " . TABLE_PREFIX . "thread AS thread ON ( thread.threadid = post.threadid ) 
    WHERE thread.forumid IN ( 1, 2, 3 ) 
    ORDER BY attachment.dateline DESC"

which finds attachment ids from forums 1, 2, or 3, newest ones first. I'm really just an SQL beginner so I don't know how you would do what the OP wants in one query. I suppose you could query all the attachments and read them until you find one from each forum, but i don't know how you'd do a query to return exactly one from each forum.