I’m not sure I understand your problem correctly. Couldn`t you log in to the gnome classic in the login window?
Are you using a free or license version which offers the creation of virtual desktops, e.g. Terminal Server?
If you don’t want to run the default desktop environment on the virtual session (in your case – GNOME 3), administrators can change the command for start a new desktop.
For GNOME classic, they have to set in node.cfg:
DefaultDesktopCommand “env GNOME_SHELL_SESSION_MODE=classic gnome-session –session gnome-classic”
All information can be found in this article https://www.nomachine.com/AR04K00667
If you don’t have administrator privileges you can run GNOME classic desktop using custom session instead of virtual desktop.
In the custom session selection window, enter following command:
env GNOME_SHELL_SESSION_MODE=classic gnome-session –session gnome-classic