ubuntu 12.04.1 desktop - install gnome3

add gnome repository for updated version:
    sudo add-apt-repository ppa:gnome3-team/gnome3

update sources database:
    sudo apt-get update

install gnome3:
    sudo apt-get install gnome-shell

restart system and select gnome on login window:
    sudo reboot

to make GNOME the default window manager when you login:
    sudo nano /etc/lightdm/lightdm.conf

change the:
    user-session=ubuntu
value to:
    user-session=gnome