The /etc folder on these Linux installations contain a file named inittab - mind you no extensions to the filename. This file contains lots of important parameters & runlevel configurations that the kernel reads while booting and configures the system accordingly. Open this file in your favourite editor and scroll down just a little bit say, till the 18th-20th line (usually) till you find something similar to this:

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# Default runlevel. The runlevels used by RHS are:

# 0 - halt (Do NOT set initdefault to this)

#

id:3:initdefault:

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