Oracle 9: Convert date from mm/dd/yyyy hh:mm:ss format to iso8601 formatted datetime
You should use SYSTIMESTAMP instead of SYSDATE, in order to get fractional seconds (which you desire as I assume).

SYSTIMESTAMP returns data type TIMESTAMP WITH TIME ZONE, thus you don't have to specify explicitly.

So it is:

SELECT
    TO_CHAR(
        SYSTIMESTAMP AT TIME ZONE 'UTC',
        'yyyy-mm-dd"T"hh24:mi:ss.ff3"Z"'
    )
FROM dual