What are the three most important parts in Fahrenheit 451? Why?

This will be highly subjective as each reader will have
their own moments that represent importance and relevance.  For me, one of the most
important moments happens early when Clarisse asks Montag is he is happy.  It is a
moment that defines Montag and his relationship with his own sense of self and world at
that instant, but also helps to configure what this will be in the future.  Mildred's
ingesting the sleeping pils is another important moment in the novel for me because it
shows Montag that something is not right in this social setting.  The fact that his
wife, who shows little signs of emotional connection in general, has to "escape" with
sleeping pills and the nonchalant attitude of the men who tend to her, indicating that
such behvaior is common, tells Montag that his hunches to doubt the world are valid. 
The confrontation with Beatty in which Montag kills his boss is signfiicant because it
represents the full transformation of Montag from obedient servant to active participant
and helps to differentiate him from the person he was into the person he will
become.