Get the Average Value Across the Columns by Using the AVG() Function

CREATE TABLE tblSales (
MonthNumber int,
MonthName varchar(10) PRIMARY KEY,
ProductA INT,ProductB INT,ProductC INT
)
 
INSERT INTO tblSales 
Select 1,'Jan',5800,5900,5400 union all
Select 2,'Feb',5750,5950,5500 union all
Select 3,'Mar',6200,5700,7100 union all
Select 4,'Apr',6300,5900,5400 union all
Select 5,'May',5350,5650,5500
 
SELECT A.*,X.AvgSales FROM tblSales A inner join
(
Select MonthName,Avg(Sales) AvgSales from
(
select MonthName,'A' Product,ProductA Sales FROM tblSales
union all
select MonthName,'B' Product,ProductB Sales FROM tblSales
union all
select MonthName,'C' Product,ProductC Sales FROM tblSales
) A group by MonthName
)X
on a.MonthName=x.MonthName  order by a.monthnumber