If F(n) is not a constant, then you have the problem that you have three independent variables and one dependent variable. That would require 4 dimensional plots.
The closest you can get to 4 dimensional plots is to use isosurface() or slice() .
Fn = @(x1, x2, x3) x1.^2 - 2*x2.^3 + 3*x3/5 + 4;
[X1, X2, X3] = meshgrid(linspace(-2,2,25));
for level = -13:5:25; isosurface(X1, X2, X3, Z, level); end