SELECT 
  * 
FROM 
  cscart_promotions as p 
  LEFT JOIN cscart_promotion_descriptions as d ON p.promotion_id = d.promotion_id 
  AND d.lang_code = 'en' 
  INNER JOIN cscart_ult_objects_sharing ON (
    cscart_ult_objects_sharing.share_object_id = p.promotion_id 
    AND cscart_ult_objects_sharing.share_company_id = 1 
    AND cscart_ult_objects_sharing.share_object_type = 'promotions'
  ) 
WHERE 
  p.promotion_id = 748

Query time 0.00027

JSON explain

{
  "query_block": {
    "select_id": 1,
    "const_condition": "1",
    "table": {
      "table_name": "p",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["promotion_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "d",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["promotion_id", "lang_code"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "cscart_ult_objects_sharing",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "159",
      "used_key_parts": [
        "share_object_id",
        "share_company_id",
        "share_object_type"
      ],
      "ref": ["const", "const", "const"],
      "rows": 1,
      "filtered": 100,
      "using_index": true
    }
  }
}

Result