Perfect.... I got the result now.....Thanku again Sean...
select
Customer.CustomerID, (case when Customer.CustomerID >3 then Orders.PONumber else cast(Orders.OrderID AS varchar) end) as 'PONumber/OrderID'
from
Customer
inner join CustomerOrders on Customer.CustomerID =CustomerOrders.CustomerID
inner join Orders on CustomerOrders.OrderID =Orders.OrderID
--Pra:-):-)--------------------------------------------------------------------------------