select
  a.sn,count(c.ip)
from
  a,b,c
where
  A.sn=B.index and B.ip=C.ip
group by
  a.sn

------解决方案--------------------
SQL code
select a.sn,c.cnt
from a
join b on A.sn=B.[index]
join (select ip,count(1) as cnt from c group by ip) c on b.ip=c.ip
group by a.sn,c.cnt

------解决方案--------------------
SQL code

select
  a.sn,count(c.ip)
from
  a,b,c
where
  A.sn=B.index and B.ip=C.ip
group by
  a.sn

------解决方案--------------------
SQL code
select
  a.sn,count(c.ip)
from a
inner join b on A.sn=B.index 
inner join c on B.ip=C.ip
group by a.sn