I hope this query will do the job
Code:
SELECT CustName, Name, SumQuatations, SumOrders, SumSales
FROM (((
(SELECT CustProd.CustID, CustProd.ProductID, SumQuatations
FROM (SELECT CustID, ProductID FROM Quotation UNION SELECT CustID, ProductID FROM Orders UNION SELECT CustID, ProductID FROM Sales) AS CustProd
LEFT JOIN (SELECT Customer.CustID, TblProducts.ProductID, Sum(Quantity) AS SumQuatations
FROM TblProducts INNER JOIN (Customer INNER JOIN Quotation ON Customer.CustID = Quotation.CustID) ON TblProducts.ProductID = Quotation.ProductID
GROUP BY Customer.CustID, TblProducts.ProductID) AS SumQuotations
ON (CustProd.CustID = SumQuotations.CustID) AND (CustProd.ProductID = SumQuotations.ProductID)) QQ
INNER JOIN
(SELECT CustProd.CustID, CustProd.ProductID, SumOrders
FROM (SELECT CustID, ProductID FROM Quotation UNION SELECT CustID, ProductID FROM Orders UNION SELECT CustID, ProductID FROM Sales) AS CustProd
LEFT JOIN (SELECT Customer.CustID, TblProducts.ProductID, Sum(Quantity) AS SumOrders
FROM TblProducts INNER JOIN (Customer INNER JOIN Orders ON Customer.CustID = Orders.CustID) ON TblProducts.ProductID = Orders.ProductID
GROUP BY Customer.CustID, TblProducts.ProductID) AS SumOrders
ON (CustProd.CustID = SumOrders.CustID) AND (CustProd.ProductID = SumOrders.ProductID)) QO
ON (QQ.CustID = QO.CustID) AND (QQ.ProductID = QO.ProductID))
INNER JOIN
(SELECT CustProd.CustID, CustProd.ProductID, SumSales
FROM (SELECT CustID, ProductID FROM Quotation UNION SELECT CustID, ProductID FROM Orders UNION SELECT CustID, ProductID FROM Sales) AS CustProd
LEFT JOIN (SELECT Customer.CustID, TblProducts.ProductID, Sum(Quantity) AS SumSales
FROM TblProducts INNER JOIN (Customer INNER JOIN Sales ON Customer.CustID = Sales.CustID) ON TblProducts.ProductID = Sales.ProductID
GROUP BY Customer.CustID, TblProducts.ProductID) AS SumSales
ON (CustProd.CustID = SumSales.CustID) AND (CustProd.ProductID = SumSales.ProductID)) QS
ON (QQ.CustID = QS.CustID) AND (QO.ProductID = QS.ProductID))
INNER JOIN Customer ON QQ.CustID = Customer.CustID)
INNER JOIN TblProducts ON QQ.ProductID = TblProducts.ProductID;