Oracle SQL insert into with With clause

You may use as many 'helper_tables' as you wish.

create table t(helper1 varchar2(50) , helper2 varchar2(50) , dataElement varchar2(50) );


insert into t(helper1, helper2, dataelement)
with
     de as(select level lvl from dual connect by level <10)
     ,h1 as (select lvl, lvl/1.5 hp from de)
     ,h2 as (select lvl,  lvl/2 hp2 from de)
select h1.hp , h2.hp2, de.lvl
  from de 
        inner join
       h1 on de.lvl = h1.lvl
        inner join
       h2 on de.lvl = h2.lvl
/

With this in mind, you may be able to do all of your joins via normal joining of the tables to the master table

INSERT
INTO    dummy1
WITH    helper_table AS
        (
        SELECT  *
        FROM    dummy2
        )
SELECT  t.a
FROM    helper_table t
WHERE   t.a = 'X'

You can do something like

INSERT INTO dummy1
  WITH helper_table AS (
    SELECT *
      FROM dummy2
    )
  SELECT t.a
    FROM helper_table t
   WHERE t.a = 'X';

For your updated query

INSERT
INTO    dummy values (a,b)  //more values
WITH    helper_table AS
    (
    SELECT  *
    FROM    dummy2
    ),
        helper_table2 AS   //from more tables
    (
    SELECT  *
    FROM    dummy3
    )         
SELECT  t.value as a, t2.value as b
FROM    helper_table t 
join helper_table t2 on t.value = t2.value //some join
WHERE   t.value = 'X' and t2.value = 'X'   //other stuff