CREATE TABLE aa (a int primary key);
CREATE TABLE bb (a int primary key references aa);
CREATE TABLE cc (a int primary key references bb);

SELECT union2.a
FROM (SELECT union1.a
      FROM (SELECT aa.a FROM aa
            UNION ALL
            SELECT bb.a FROM bb) AS union1
      UNION ALL
      SELECT cc.a FROM cc) AS union2
 INNER JOIN aa ON union2.a = aa.a;