OK I'm a linux newbie, I've installed the rpm's for KDE3 so what do I do next, to be able to use it. :-[
Is KDEDIR=/path/to/the/new/kde/dir ?
Is /path/to/the/new/kde/dir/bin in your path ?
is ~/.xinitrc set to run startkde ?
that way startx should automatically run KDE3 when issued - I don't know much about KDM so I can't help you wil that ?