Lots of people are wanting to install Open Office 3 on Ubuntu Hardy including some of my family members. I'll keep this sweet and short and assume that you already know a little bit about Ubuntu.
1) Go to System>Administration>Software Sources
2) Add this to the sources list:
deb http://ppa.launchpad.net/openoffice-pkgs/ubuntu hardy main
3) Let the sources list applet reload and update
4) Run the Ubdate Manager from; System>Administration>Update Manager
5) Install the updates
6) Enjoy Open Office 3 on your Hardy installation