select t.number
from 
(select *,rank()over(order by number) as rank2 from grade) t
group by t.rank2 
having count(1)>=3

先用窗口函数对积分进行排名，相同的积分名次相同。再以排名作为关键字段进行分组，统计每组中记录次数大于等于3的，即为所求。