oracle left outer joins not showing right null values

table myTable(refnum, contid, type)

values are:
1, 10, 90000
2, 20, 90000
3, 30, 90000
4, 20, 10000
5, 30, 10000
6, 10, 20000
7, 20, 20000
8, 30, 20000

select a.refnum from myTable a where type = 90000
select b.refnum from myTable b where type = 10000 and contid in (select contid from myTable where type = 90000)
select c.refnum from myTable c where type = 20000 and contid in (select contid from myTable where type = 90000)

a.refnum, b.refnum, c.refnum

select a.refnum, b.refnum, c.refnum
from myTable a
left outer join myTable b on (a.contid = b.contid)
left outer join myTable c on (a.contid = c.contid)
where a.id_tp_cd = 90000
and b.id_tp_cd = 10000
and c.id_tp_cd = 20000

1, null, 6
2, 4, 7
3, 5, 8

2, 4, 7
3, 5, 8

and b.id_tp_cd = 10000
and c.id_tp_cd = 20000

select a.refnum, b.refnum, c.refnum
from myTable a
left outer join myTable b on (a.contid = b.contid and b.id_tp_cd = 10000)
left outer join myTable c on (a.contid = c.contid and c.id_tp_cd = 20000)
where a.id_tp_cd = 90000

select a.refnum, b.refnum, c.refnum
from myTable a, mytable b, mytable c
where a.contid=b.contid(+)
and a.contid=c.contid(+)
and a.type = 90000
and b.type(+) = 10000
and c.type(+) = 20000;


REFNUM     REFNUM     REFNUM
---------- ---------- ----------
     1                     6
     2          4          7
     3          5          8