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.