First open ur terminal
- sudo add-apt-repository ppa:gnome3-team/gnome3
- sudo add-apt-repository ppa:ricotz/testing
- sudo add-apt-repository ppa:ricotz/staging
- sudo apt-get update && sudo apt-get upgrade
- sudo apt-get install gdm gnome-shell gnome-tweak-tool
- Reboot and type gnome-shell --version and WHOOPSSS u have gnome 3.8 on ur ubuntu 12.04 ;)