●MySQL 3つ以上のテーブル結合

select 表示列名...
 from 

 ( table1 inner join table2 
     on table1.id = table2.id )

 inner join table3 
     on table1.id = table3.id;

-----------------------------------
[参考SQL文]

SELECT
  order_main.po_id as 注文書番号,
  usr.l_name,
  usr.f_name,
  order_desc.p_id as 商品番号,
  p_name as 商品名,
  order_desc.quantity as 数量 

#注文書テーブルとユーザテーブルの結合
FROM
  order_main
INNER JOIN
  usr
ON
  order_main.user_id = usr.user_id 

#明細書テーブルの結合
INNER JOIN 
  order_desc 
ON
  order_main.po_id = order_desc.po_id 

#商品テーブルの結合
INNER JOIN 
  product  
ON
  product.p_id = order_desc.p_id   
;