When you run gnome-tweak-tool after installing Gnome 3 you can get the following error:
GLib-GIO-ERROR **: Settings schema 'org.gnome.shell.clock' is not installed
The cause is that you are missing a schema which is installed together with gnome-shell (which is probably not installed because you are running the fallback mode). The obvious fix is to install gnome-shell. But if you want to run Gnome 3 in fallback mode you need to do some additional steps: Open the System Settings, go to System Info, Graphics and check “Forced Fallback Mode”.