select t.* from (select x from A
union
select x from B
union
select x from C) t
where t.x not in (select a.x from A a , B b where a.x = b.x)
and t.x not in (select a.x from A a ,C c where a.x = c.x )
and t.x not in (select b.x from B b , C c where b.x = c.x)