SELECT 
  r.rating_value, 
  COUNT(b.post_id) as total_items 
FROM 
  cscart_discussion as a 
  LEFT JOIN cscart_discussion_posts as b ON a.thread_id = b.thread_id 
  LEFT JOIN cscart_discussion_rating r ON r.post_id = b.post_id 
WHERE 
  a.object_type = 'M' 
  AND b.status = 'A' 
  AND a.object_id = 3 
GROUP BY 
  r.rating_value

Query time 0.00017

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "message": "Impossible WHERE noticed after reading const tables"
    }
  }
}