Note that this is an unofficial way to install GNOME 3 on Ubuntu 11.04 (Natty Narwhal), things might be broken, so proceed with caution.
Step 1: Add GNOME 3 PPA repository
sudo apt-add-repository ppa:gnome3-team/gnome3
Then update the apt-get repository database by running:
sudo apt-get update
Step 2: Install the Gnome 3 Desktop environment using this command
sudo apt-get install gnome-desktop3
Note that this might screw up your installation, you’ve been warned.