Howto set GNOME3 as the default WM in tightvnc-server?

Hello friends !
I need some help with tightvnc-server. I configured it to share a specific user desktop by following the method 2 described here.
It works but I want to set the default GNOME3 interface -the same one as the console- instead of gnome-flashback-metacity as defined in /usr/share/defaults/xinit/xinitrc .
What should I do, please ?
Best regards