Different default Display Managers for different users

I'm using Kubuntu and have installed Gnome and Xfce and Fluxbox.
I want to set the computer up to load a different manager for each different user (as default) when they login.
e.g. I use XFCE, my wife Gnome, and my mother-in-law KDE.
I'm not sure how to set this up - are there files in each user directory that can specify the default display manager?

First off, make sure you have KDM set up to load multiple WMs: http://docs.kde.org/kde3/en/kdebase/kdm/different-window-managers-with-kdm.html

Then you can change each user's .kdmrc file to each use a different session:http://docs.kde.org/kde3/en/kdebase/kdm/kdm-scripts.html#kdmrc-xsession