方法1：使用外表与内表连接，取出不小于3的number，最后对number去重
SELECT DISTINCT(a.number)
FROM grade AS a
WHERE (
SELECT COUNT(*)
FROM grade AS b
WHERE a.number=b.number
)>=3

方法2：按照number分组，然后取出组大小不小于3的组的number
SELECT g.number
FROM grade AS g
GROUP BY g.number
HAVING COUNT(g.id)>=3