Under Red Hat Enterprise Linux, it is possible to change the default runlevel at boot time.
To change the runlevel of a single boot session, use the following instructions:
When the GRUB menu bypass screen appears at boot time, press any key to enter the GRUB menu (within the first three seconds).
Add <space><runlevel> at the end of the boot options line to boot to the desired runlevel. For example, the following entry would initiate a boot process into runlevel 3:
grub append> ro root=/dev/VolGroup00/LogVol00 rhgb quiet 3