At the end of Act III, what does John Proctor's quote try to say to the audience?Proctor [laughs insanely, then]: A fire, a fire is burning! I...

Proctor's closing to Act III says a couple of things to
the audience. The first and most essential is that Proctor is reaching a point where his
statement of dissent cannot be quelled.  For a period of time, Proctor was quite content
with not saying anything and allowing what was going on to continue without spoken
opposition.  Given the hypocrisy he saw in the courtroom and the fact that innocent
people were being railroaded by a system that professes truth but actually operates in
opposite, Proctor cannot be silent.  Another element that is coming forth to the
audience is the idea that Proctor is now actively embracing self- destruction.  Proctor
tried for a period to try to keep some level of protection regarding his interests. 
However, as the net of inclusion in the accusations was widened, Proctor found it more
difficult to remain distinct.  He protests in the courtroom, stands against Parris,
confesses to lechery, nearly chokes the life out of Abigail, and then the declaration
that "God damns our kind."  There is a gradual progression of self- destruction,
uncontrollable in its timbre and reflective to the audience that Proctor is enduring
something awful, where death is going to be present itself.  Finally, I think that
Proctor's statement about religion cannot avoided.  Proctor has no problem indicting the
institution of religion that is being practiced in Salem.  In the implication that
hypocrisy will end up being punished, Proctor makes a statement that the nature of
organized religion and the legal system in Salem is one where there is hypocrisy,
something that Proctor is convinced will be brought up and will be proven in some form. 
Regardless of the damage it does to Proctor, it is going to be demonstrated and this is
something that the audience is told through his closing.