I'd like to start-up Xserves at Unix run level 3. At least turn off the GUI and have all the service app (networking, ssh, apache, mysql, etc.) running in the background. Is this possible? How do I configure this?
OS X does not have runlevels. It natively boots into the GUI. If you want you can set the server to not auto-login and just list the username and password fields on boot. From there you can type ">console" in the user field and press return. This will exit the GUI and drop you into a shell.
May I ask what you want to accomplish by booting into "runlevel 3"?