Contours can be drawn on a 2D slice in a 3D figure using contourslice.
I'm not sure what the contours are that you want to plot. I just specify some random contour in variable V.
Note that you are not projecting points at the wall, but at the location of the maximum. I fixed this by actually plotting at the limits.
x = mvnrnd(1, 3, 100);
y = mvnrnd(2, 3, 100);
z = mvnrnd(3, 3, 100);
scatter3 (x, y, z, 'filled')
hold on
xl = xlim;
yl = ylim;
zl = zlim;
xlim(xl); ylim(yl); zlim(zl); % fix the limits for if we rotate
scatter3 (0*x+xl(2), y, z, '.')
scatter3 (x, 0*y+yl(2), z, '.')
scatter3 (x, y, 0*z+zl(1), '.')
rotate3d on
[X,Y,Z]=meshgrid(round(xl(1)):round(xl(2)),round(yl(1)):round(yl(2)),round(zl(1)):round(zl(2)));
V = X.^2+Y.^2+Z.^2;
contourslice(X,Y,Z,V,xl(2),yl(2),[zl(1) zl(2)])
hold off
