Your approach should work, doesn't it?

Maybe this could be a better performance:

SELECT

MAX(

CASE WHEN field1 > field2 THEN

CASE WHEN field1 > field3 THEN

field1

ELSE field3 END

ELSE

CASE WHEN field2 > field3 THEN

field2

ELSE field3 END

END

)

FROM myTable

Greets

Flo