DISTINCT on one column and return TOP rows
Replace your dbname and schemaName in the following query.
;WITH CTE AS
(
SELECT
[Order_No]
,[Customer_Name]
,[Purchase_Cost]
, ROW_NUMBER() OVER(PARTITION BY [customer Name] ORDER BY [Purchase Cost] DESC) AS "RowNumber"
FROM [dbname].[schemaName].[PurchaseTable]
)
SELECT TOP(3)
[Order_No]
,[Customer_Name]
,[Purchase_Cost]
FROM CTE WHERE RowNumber=1
ORDER BY [Purchase_Cost] DESC
I am sure there are other ways of doing the same. I suggest you read this.