Depending on what you're trying to achieve...
1) If you want to get max of all columns in all rows:
SELECT MAX(C123) AS MaxValue
FROM (
SELECT C1 AS C123
FROM @tmp
UNION ALL
SELECT C2 AS C123
FROM @tmp
UNION ALL
SELECT C3 AS C123
FROM @tmp
) AS T
2) If you want to get max value of each column (in a single row):
SELECT MAX(C1) AS C1Max, MAX(C2) AS C2Max, MAX(C3) AS C3Max
FROM @tmp
3) If you want to get max value of each column (in rows with column names):
SELECT ColName, MAX(ColValue) AS MaxValueOfCol
FROM (
SELECT ColName, ColValue
FROM (
SELECT C1, C2, C3
FROM @tmp
) AS PVT
UNPIVOT(ColValue FOR ColName IN([C1], [C2], [C3])) AS UNPVT
) AS T
GROUP BY ColName
Note: replace
@tmp
with the name of table in your database.