you are right asking this as there seems to be a problem, at least if you replace gdm3 with lightdm. The installation and reboot with login went fine, switching to rc3 went fine, too. But it seems X is not disabled (alt-F7 still shows X). While switching to rc3 init (or whatever is acting in this moment) tries to stopp gdm3 although lightdm is installed and working. So there seems to be a glitch ...