This came out way off on the resulting Quantity for "QtyIn". It is in reality 221 for a specific customer, but the SQL statement made it come out as 4 times that value, meaning 884.

Perhaps you are correct in your statement that 3 tables will give the wrong values in a SQL Statement. The statement seems logical, but it came out wrong in its sum of the QtyIn value. Here's the code I ended up using which is a summation of several of those who contributed here:

SELECT ;
      CustNo, Customer, QtyIn, QtyOut, QtyIn-QtyOut As QtyDiff ;
   FROM ( ;
      SELECT ;
            cs.CustNo, ;
            cs.Customer, ;
            SUM(NVL(si.StandsIn,0)) AS QtyIn, ;
            SUM(NVL(so.SerialCnt,0)) As QtyOut ;
         FROM Customer cs ;
         LEFT JOIN StandsIn si ON si.cust = cs.custno ;
         LEFT JOIN Stands_Out so ON so.cust = cs.custno ;
         GROUP BY ;
            cs.CustNo, ;
            cs.Customer ;
   ) dt1