不能在select子句中使用别名，必须重复代码
你在table1.cost+之前漏掉了一个逗号。。。
在subselect中，外部表不在范围内，因此应该使用适当的join with subquery for sum

SELECT 
    table1.cost, 
    t.expensetot,
    table1.cost + t.expensetot as grandtotal,
    table3.label
FROM table1
INNER JOIN (
  select table2.key, sum(table2.expense) expensetot
  from table2
  group by  table2.key
) t on t..key=table1.id 
LEFT JOIN table3 ON table3.key=table1.id
  WHERE
    table1.saledate>SUBDATE(CURDATE(), INTERVAL 1 YEAR)
  ORDER BY grandtotal

也许这会有帮助

SELECT 
      table1.cost,
      (SELECT SUM(expense) FROM table2 WHERE table2.key=table1.id) as expensetot,
      (table1.cost+(SELECT SUM(expense) FROM table2 WHERE table2.key=table1.id)) as grandtotal,
      table3.label
    FROM
      table1
      LEFT JOIN table3 ON table3.key=table1.id
    WHERE
      table1.saledate>SUBDATE(CURDATE(), INTERVAL 1 YEAR)
    ORDER BY grandtotal

也可以阅读查询中的用户变量：https://dev.mysql.com/doc/refman/8.0/en/user-variables.html