Given that Gnome-Shell ya it is available in the repositories de Debian Testing, I started looking for the repositories of LMDE and I am pleasantly surprised that both in the NEU , as in the Incoming, there are already packages related to Gnome 3.
Right now I'm not using LMDE, but I suppose that with only updating we can install the packages Gnome-Shell. The procedure through the terminal must be the same as with Debian Testing:
$ sudo aptitude update
$ sudo aptitude upgrade
$ sudo aptitude install gnome-shell gnome-session gnome-session-fallback
Or we could use the Update Manager to see if it recommends the update.