select
from 表1 a
inner join 表2 b on a.A=b.A
left join 表3 c on b.x=c.x
where c.x is null

------解决方案--------------------

你是一a表为标准，还是ab表为标准？

------解决方案--------------------
select x from 表3 c where x not in
(select x from 表2 a left join 表1 b on a.a=b.a)