Here is a modified version of the previous approach. Add this to your init scripts (I bet it's missing!):
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#
#the same like XF86Config but for gdm.conf & Xservers
#
for i in xdm/Xservers gdm/gdm.conf; do
if [ -L "/etc/X11/$i" ]; then
DumbCon=`sed -n 's/.*dumbcon=\([0-9]*\).*/\1/p' /proc/cmdline`
[ -n "$DumbCon" ] || DumbCon=0
[ -r "/etc/X11/$i.$DumbCon" ] && ln -sf "/etc/X11/$i.$DumbCon" "/etc/X11/$i"
fi
done
echo "Setting up display managers for `expr $DumbCon + 1` Xservers"
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This will adjust the proper /etc/X11/xdm/Xservers and /etc/X11/gdm/gdm.config according to the boot line argument dumbcon=n (remember n+1= number of X users/sessions).
You have to create the configuration files following these assumptions:
"i" only stands for /etc/X11/xdm/Xserver and /etc/X11/gdm/gdm.conf.
"i.0" is used for a single X server, when dumbcon=n is not specified, or dumbcon=0.
"i.1" is used by the display manager when dumbcon=1 is specified.
"i.2" is used by the display manager when dumbcon=2 is specified.
"i.3" is used by the display manager when dumbcon=3 is specified.
...and so on.
Therefore:
"i.0" should contain the definition only of your original standard X server.
"i.1" should contain the definitions for 2 X servers.
"i.2" should contain the definitions for 3 X servers.
...and so on.
If you boot without dumbcon=n or with dumbcon=0 (for example a standard kernel), your display manager will start a single X server with the corresponding XF86Config file.
If you start with dumbcon=1 the display manager will automatically start 2 X servers.
If you start with dumbcon=2, when booting is finished you'll get 3 login prompts on your 3 displays.
Keep in mind that each X server should have it's own configuration file, and it should be specified in the display manager configuration file properly. Take a look at the configuration files before restarting with an activated display manager and this addition to your init scripts.
This can also be used if you have a single XFree configuration file (see Section 4.6, "For graphic cards without DRI"). In this case you will have to specify the correct layout instead of the correct XFree configuration file.