From the manpages:
Runlevels are a concept from UNIX(R) System V used by the init(8)
daemon or other system initialisation system to define modes of system
operation.
The Upstart init(8) daemon has no native concepts of runlevel, and
unlike the System V daemon, makes no attempt to keep track of the
current runlevel.
Ubuntu and other modern Linux distributions have replaced System V init
with Upstart:
Upstart is an event-based replacement for the /sbin/init daemon which handles starting of tasks and services during boot, stopping them during shutdown and supervising them while the system is running.
If you must switch to what is now closest to runlevel 3
do it by stopping your gdm
or lightdm
service.