OpenOffice 3.0 is not integrated in Ubuntu 8.10. Normally you have to remove the OpenOffice 2.x paket and manually install the version 3.
The alternative way is to add this line to your software sources: (/etc/apt/sources.list)
deb http://ppa.launchpad.net/openoffice-pkgs/ubuntu intrepid main
Now you can upgrade to version 3 with the paket manager with this commands:
aptitude update aptitude upgrade
Read this article in englisch.