select aid ,round(avg(num),2)
         from ( 
                  select  aid ,num,
                           row_number() over(partition by aid order by num) rn 
                  from tb1
                )t1 
          where rn < 4
group by num

------解决方案--------------------

SQL code
select aid 
avg(num)
from(select aid ,
num ,
row_number()over(partition by aid  order by num asc) rn 
from tab )
where rn<=3 
group by aid