Or like this?

--select the grandmax of the max of three columns

DECLARE @date1Max AS datetime = (SELECT MAX([orderdate])

FROM [Sales].[OrderValues]);

DECLARE @date2Max AS datetime = (SELECT MAX([requireddate])

FROM [Sales].[OrderValues]);

DECLARE @date3Max AS datetime = (SELECT MAX([shippeddate])

FROM [Sales].[OrderValues]);

--SELECT @date1Max AS orderdate, @date2Max AS requireddate , @date3Max AS shippeddate; --TO CHECK CORRECT DATE

SELECT CASE

WHEN @date1Max >= @date2Max AND @date1Max >= @date2Max THEN @date1Max

WHEN @date2Max >= @date1Max AND @date1Max >= @date3Max THEN @date2Max

ELSE @date3Max

END AS GrandMax;