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KDE3 is out of support of the openSUSE distribution but well stable. You can install 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 firstname.lastname@example.org 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).
To choose kdm3 as the default session manager one needs to install it (kdebase3-kdm) and then 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.