How can I plot 3D surface, where x(S1) ,y(S2) and z(S_Hill) are functions of other variables(Sxx, Sxy, Syy)

clear

G = 0.3261;

H = 0.5648;

F = 0.4352;

N = 1.3450;

k = 0;

Sxx = linspace(-400e6,400e6,20);

Sxy = linspace(-400e6,400e6,20);

Syy = linspace(-400e6,400e6,20);

[Sx,Sy,Sz] = meshgrid(Sxx,Syy,Sxy);

S1 = (1/2)*(Sx+Sy+((Sx-Sy).^2+4*Sz.^2).^(1/2));

S2 = (1/2)*(Sx+Sy-((Sx-Sy).^2+4*Sz.^2).^(1/2));

S_Hill = ((G+H)*Sx.^2-2*H*Sx.*Sy+(H+F)*Sy.^2+2*N*Sz).^(1/2);

surf(X,Y,Z);