I have one of my machines running kde3, via pkgsrc.  I configured it
for fast user switching by editing /usr/pkg/share/kde/config/kdm/Xservers
to this:

	# Xservers - local X-server list
	#
	# This file should contain an entry to start the server on the
	# local display; if you have more than one display (not screen),
	# you can add entries to the list (one per line).
	# If you also have some X terminals connected which do not support XDMCP,
	# you can add them here as well; you will want to leave those terminals
	# on and connected to the network, else kdm will have a tougher time
	# managing them. Each X terminal line should look like:
	#       XTerminalName:0 foreign
	#

	:0 local /usr/X11R6/bin/X -nolisten tcp vt05
	:1 local /usr/X11R6/bin/X -nolisten tcp :1 vt06
	:2 local /usr/X11R6/bin/X -nolisten tcp :2 vt07
	:3 local /usr/X11R6/bin/X -nolisten tcp :3 vt08


	### Don't change these two lines; they are hints for genkdmconf. ###
	### Version 1.99 ###

The :1, :2, and :3 lines are my additions.

(Aside: the documentation implies that this file is generated correctly
at boot time, and will have those extra lines if the system supports it.
NetBSD clearly does support it, but those lines were not installed.
PR?)

The problem is the screen resolution: I don't know how to control
what resolution the different screens are.  It seems to be unpredictable.
Right now, vt05 and vt06 appear to be 640x480 (possibly 800x600); vt07
and vt08 are at the correct 1280x1024, which is set in /etc/X11/XF86Config.