How can I find 'delta' between 3 or more surfaces?

Consider following sample surfaces:

x = [-20:20];
y = [-20;20];
z1 = x + y;
z2 = x + 2y;
z3 = x + 3y;

For a given x and y value, how can I find out which of the three above equations has a value for z that is unique to that equation for that given x and y? For example (hypothetical), for x = 1, and y = 3, z1 gave 3, z2 gave 3, but z3 gave 6, then I would want to know that z3 is different at x = 1 and y = 3.

I want to see this in either a data table form or in a graphical form of some sort. What do people here think I should do?

Thank you

[Marta Salas]

This question is similar to the one you posted some days ago. They already suggested you a solution when you have more than 2 surfaces: http://www.mathworks.com/matlabcentral/answers/122629-distinguishing-between-two-surfaces