SELECT 
  COUNT(*) 
FROM 
  cscart_discussion_posts 
  LEFT JOIN cscart_discussion_messages ON cscart_discussion_messages.post_id = cscart_discussion_posts.post_id 
  LEFT JOIN cscart_discussion_rating ON cscart_discussion_rating.post_id = cscart_discussion_posts.post_id 
WHERE 
  cscart_discussion_posts.thread_id = 26 
  AND cscart_discussion_posts.status = 'A'

Query time 0.00061

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_discussion_posts",
      "access_type": "ref",
      "possible_keys": ["thread_id", "thread_id_2"],
      "key": "thread_id",
      "key_length": "3",
      "used_key_parts": ["thread_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "(cscart_discussion_posts.`status` = 'A')"
    }
  }
}