If your ranges grow more complex, you should try to define groups so you can use splitapply. But for this simple example you can simple use logical indexing on your 2D (not 3D) data.
data = [1 -5 -2; 0 2 3; 2 -5 -2; -0.5 2 5; -0.8 3 3.5];
x = data(:,1);
y = data(:,2);
z = data(:,3);
value = sum(z( x>=-1 & x<1 & y>=1 & y<5 ));
disp(value)
