不记得用having了，搞了个窗口函数，写复杂了，不过能通过

SELECT DISTINCT g.number 
FROM grade g 
WHERE EXISTS (
    SELECT g1.number 
    FROM (
        SELECT id,number,COUNT(*)OVER(PARTITION BY number) AS cnumber
        FROM grade) AS g1
    WHERE g1.id=g.id
    AND g1.cnumber>=3)