As of openSUSE 12.1 KDE 3 desktop is included as officially supported part of the distribution. Additional KDE 3 software can be installed from community-supported KDE:KDE3 repository.
If you are interested in (helping) maintaining the KDE3 repository for openSUSE, you are welcome to do so. Contact the KDE team in the #opensuse-kde channel on Freenode or on the email@example.com mailing list.
Installing KDE 3
Easiest way to install KDE3 on any openSUSE version is to use 1-click install:
To manually install the KDE3 desktop, you will need to install kdebase3-session package - plus any of the package families that you would like (e.g. kdegames, kdegraphics etc). Under openSUSE before 11.4 you would have to add KDE:KDE3 repository, under openSUSE 11.4 onwords KDE3 is included in the main openSUSE repository. It is possible to use KDE3 packages when running a KDE4 desktop and kde4 packages when running a KDE3 desktop. So for example the music player Amarok version 1.4 (designed for KDE3 systems) will play fine when running the kde4 desktop and Amarok version 2 (designed for KDE4) will play fine when running the kde3 desktop.
To choose kdm3 as the default session manager one needs to write
DISPLAYMANAGER="kdm3" line to the file /etc/sysconfig/displaymanager (this can be done with the Yast module etc/sysconfig tool Editor as well).
There is a dedicated list for KDE3 on openSUSE.
You can also try TDE (Trinity Desktop Environment) if you have interest. TDE is on going project and has active developers.
Installation instructions for openSUSE is available here.