select a.category_no, a.regdate
from (
    select category_no,regdate
    from testTable
    group by category_no,regdate
) a inner join
(
    select category_no,regdate
    from testTable
    group by category_no,regdate
) b on a.category_no = b.category_no and a.regdate <= b.regdate
group by a.category_no, a.regdate
having count(*) <= 3
order by a.category_no, a.regdate desc;