SELECT id, 'messages' as 'table' FROM messages 
  WHERE content LIKE '%keyword%' 
    OR title LIKE '%keyword%'
UNION
SELECT id, 'topics' as 'table' FROM topics
  WHERE content LIKE '%keyword%' 
    OR title LIKE '%keyword%'
UNION
SELECT id, 'comments' as 'table' FROM comments
  WHERE content LIKE '%keyword%' 
    OR title LIKE '%keyword%'