I have Gnome installed with Debian 8. Assuming you log in through the log-in GUI, when you select the user to begin the log-in process, a cog wheel appears beside the "sign-in" button. If you click on this cog, a list of available desktops appears. Mine shows Gnome (which is Gnome3), Gnome Flashback, Gnome on Wayland, and several other choices. After you choose one of these and log in, it will remain your default choice until you click the cog again at log-in and choose something else.
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