Comparing three or more surfaces to find unique z given x and y

Hi guys,

I have asked this question before but still not sure the best way to approach this. I am basically trying to compare three formulas to figure out which formula to use in a certain scenario. In order to find out this 'best formula', for a given x and y value, I basically want to find out which formula gives me a unique value of z, that the other two formulas don't. If all three give different values, then I want to know. If all three give same value, then I don't want to know because all formulas can be used in any scenario. I want to gets this data 1) in matrix form and 2) in a surface/graph form. I have made a start, but not sure what to next:

x = [-10:10];
y = [-10:10];
z1 = x + y;
Z1 = meshgrid(z1);
z2 = 2x + y;
Z2 = meshgrid(z2);
z3 = 3x + y;
Z3 = meshgrid(z3);
It would be a lot easier if I was only looking at two equations. I could then do a simple surf(x,y,z2-z1);.

I hope I have made my question clear.

Thank you