Find the saving balance of the account with the highest checking balance.
select t3.balance from accounts as t1 join checking as t2 on t1.custid = t2.custid join savings as t3 on t1.custid = t3.custid order by t2.balance desc limit 1
And how about the lowest?
select t3.balance from accounts as t1 join checking as t2 on t1.custid = t2.custid join savings as t3 on t1.custid = t3.custid order by t2.balance limit 1
Checking balance of Customer id 3
select balance from checking where custid = '3'