SQL> ed
Wrote file afiedt.buf

  1  with t (id,element,id_parent) as (
  2  select 1,'a',null from dual union all
  3  select 2,'aa',1 from dual union all
  4  select 3,'ab',1 from dual union all
  5  select 4,'ac',1 from dual union all
  6  select 5,'aaa',2 from dual union all
  7  select 6,'aac',2 from dual union all
  8  select 7,'aad',2 from dual union all
  9  select 8,'aaaa',5 from dual union all
 10  select 9,'aaab',5 from dual union all
 11  select 10,'aaac',5 from dual union all
 12  select 11,'aba',3 from dual union all
 13  select 12,'abb',3 from dual union all
 14  select 13,'abc',3 from dual union all
 15  select 14,'aca',4 from dual union all
 16  select 15,'acb',4 from dual union all
 17  select 16,'acc',4 from dual union all
 18  select 17,'aaca',6 from dual union all
 19  select 18,'aacb',6 from dual union all
 20  select 19,'aacc',6 from dual
 21  )
 22  select id,id_parent,level,rpad(' ',(level-1)*2,' ')||element element from t
 23  start with id_parent is null
 24* connect by id_parent=prior id
SQL> /

        ID  ID_PARENT      LEVEL ELEMENT
---------- ---------- ---------- --------------------
         1                     1 a
         2          1          2   aa
         5          2          3     aaa
         8          5          4       aaaa
         9          5          4       aaab
        10          5          4       aaac
         6          2          3     aac
        17          6          4       aaca
        18          6          4       aacb
        19          6          4       aacc
         7          2          3     aad
         3          1          2   ab
        11          3          3     aba
        12          3          3     abb
        13          3          3     abc
         4          1          2   ac
        14          4          3     aca
        15          4          3     acb
        16          4          3     acc

19 rows selected.

with t (id,element,id_parent) as (
select 1,'a',null from dual union all
select 2,'aa',1 from dual union all
select 3,'ab',1 from dual union all
select 4,'ac',1 from dual union all
select 5,'aaa',2 from dual union all
select 6,'aac',2 from dual union all
select 7,'aad',2 from dual union all
select 8,'aaaa',5 from dual union all
select 9,'aaab',5 from dual union all
select 10,'aaac',5 from dual union all
select 11,'aba',3 from dual union all
select 12,'abb',3 from dual union all
select 13,'abc',3 from dual union all
select 14,'aca',4 from dual union all
select 15,'acb',4 from dual union all
select 16,'acca',15 from dual union all
select 17,'aaca',6 from dual union all
select 18,'aacb',6 from dual union all
select 19,'aacc',6 from dual 
)
select t.* from t
where exists (
  select 1 from t t2
  where level>2
  start with t2.id=t.id 
  connect by t2.id_parent=prior t2.id and level<=3
)
/