Open Terminal ( Default shortcut is : CTRL+ALT+T ) and Type the following commands (Make sure that you haven’t opened Ubuntu Software Center or Synaptic Package Manager while installing package from the command line).

Warning : Installing Gnome 3.0 on Ubuntu 11.04 using this PPA, will break Unity and your system might become unusable.

sudo add-apt-repository ppa:gnome3-team/gnome3
sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get install gnome-shell

Then Restart your computer; select the GNOME 3 desktop at login prompt (Default is Ubuntu, which means Unity).

To Remove

sudo ppa-purge ppa:gnome3-team/gnome3</pre>

thanks to blog.sudobits.com