It might not seem obvious, but the function is isosurface. You've got a function defined in a 3D space and you're looking for the surface where that's equal to a constant. That surface is called an "isosurface". It's like the 3D analogue of a contour in 2D.
Here's an example:
[y,x,z] = ndgrid(linspace(-5,5,64));
f = (y-z).^2+(z-x).^2+(x-y).^2;
r = 1;
isosurface(x,y,z,f,3*r^2)
r = 2;
isosurface(x,y,z,f,3*r^2)
r = 3;
isosurface(x,y,z,f,3*r^2)
