Re: xfce or lxde
As you already have Lxde and Xfce installed all you need to do is click ' log out ' in Gnome - sorry I can't be more specific, my Gnome desktop is stuck in fallback mode - and when the log-in screen comes up click on your user as usual. The screen then asks for your password, before entering that look at the bottom of the log-in panel, there should be a window showing ' Gnome ' with an arrow beside it. Click the arrow and it will open a drop-down menu showing all the desktop environments you have installed, just select the one you want to try. ( The log-in screen may just show ' other ', in which case click it and enter your username when prompted. )
As I said, my Gnome is stuck in fallback at present, but I did notice an option to force fallback, the icon looks like a slider switch, but I can't help with where it's found in full Gnome 3.
Hope this has been of some help.
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Desktop #1 F18
Desktop #2 Mint 14
Laptop: Macpup 529
Netbook: Debian ARM