If Debian is like most linux's you can enter 'init 3' as root to enter init level 3.
Check out the contents of /etc/inittab to make certain.
Mine contains these lines (among others):
# The default runlevel is defined here
# First script to be executed, if not booting in emergency (-b) mode
# /etc/init.d/rc takes care of runlevel handling
# runlevel 0 is System halt (Do not use this for initdefault!)
# runlevel 1 is Single user mode
# runlevel 2 is Local multiuser without remote network (e.g. NFS)
# runlevel 3 is Full multiuser with network
# runlevel 4 is Not used
# runlevel 5 is Full multiuser with network and xdm
# runlevel 6 is System reboot (Do not use this for initdefault!)
The 'id:3:initdefault' is where the default level is determined. On my system, to boot into xwindows, I would change the line to read: id:5:initdefault
There may also be an 'init s' for single on your system. Remember that some distro's use slightly different runlevel values, so be sure to check the file.