You're going to have to reshape your data to do this in Tableau so that instead of having 3 columns for the X, Y, Z measures, there is 1 column for the measure and another column that is a "Measure Name" dimension that identifies X, Y, Z. Then you can have a calc field that is something like IF SUM([Measure Value]) < 2 THEN "Red" ELSEIF SUM([Measure Value]) > 4 THEN "Green" ELSE "Black" END
Tableau provides a data reshaper plug-in for Excel to do this, see Installing the Tableau Add-In for Reshaping Data in Excel | Tableau Software.
The reason why you need to reshape your data is that Tableau does not provide us access to the Measure Names/Measure Values "dimension" and "measure" via calculated fields. There are a couple of Ideas to change this, vote them up if you are interested: http://community.tableau.com/ideas/1071 and http://community.tableau.com/ideas/1187.
Thank you Jonathan.
I will try that out.