select 
count(*) 
from
(select
t1.sId
from
(select
sId,
case
    when cId=1 then score else 0 end as c1_score
from SC) t1
left join 
(select
sId,
case
    when cId=2 then score else 0 end as c2_score
from SC) t2 on t1.sId=t2.sId 
left join 
(select
sId,
case
    when cId=3 then score else 0 end as c3_score
from SC) t3 on t2.sId=t3.sId
group by t1.sId
having avg(t1.c1_score+t2.c2_score+t3.c3_score)>60) t4