I suggest you use TO_CHAR()
when converting to string. In order to do that, you need to build a date first.
SELECT TO_CHAR(TO_DATE(DAY||'-'||MONTH||'-'||YEAR, 'dd-mm-yyyy'), 'dd-mm-yyyy') AS FORMATTED_DATE
FROM
(SELECT EXTRACT( DAY FROM
(SELECT TO_DATE('1/21/2000', 'mm/dd/yyyy')
FROM DUAL
)) AS DAY, TO_NUMBER(EXTRACT( MONTH FROM
(SELECT TO_DATE('1/21/2000', 'mm/dd/yyyy') FROM DUAL
)), 09) AS MONTH, EXTRACT(YEAR FROM
(SELECT TO_DATE('1/21/2000', 'mm/dd/yyyy') FROM DUAL
)) AS YEAR
FROM DUAL
);