with chars as
 (select case
           when level > 10 then
            chr(54 + level)
           else
            to_char(level - 1)
         end as letter
    from dual
  connect by level <= 36),
codes as
 (select c2.letter  c1.letter  c0.letter as code
    from chars c0, chars c1, chars c2)
select *
  from codes t
 where not exists (select 1 from mytbl f where t.code = f.code)
   and t.code between (select min(code) from mytbl) and (select max(code) from mytbl);