What is the theme of part 3 of Fahrenheit 451?
Part 3 is titled "Burning Bright," and is the culmination of all the previous events. Montag, having come to the conclusion that society is diseased from within and must be changed, is forced to consider the changes in himself as well as the anger shown to him by society. Instead of living peacefully within society, Montag is now thinking individually and pushing against the system; his passions, which he still does not fully understand, are "burning bright" among the sameness of all the other citizens.
Lights flicked on and house-doors opened all down the street, to watch the carnival set up. Montag and Beatty stared, one with dry satisfaction, the other with disbelief, at the house before them, this main ring in which torches would be juggled and fire eaten.
"Well," said Beatty, "now you did it. Old Montag wanted to fly near the sun and now that he's burnt his damn wings, he wonders why."
(Bradbury, Fahrenheit 451, hrsbstaff.ednet.ns.ca)
The overall theme is Montag's movement, his final abandonment of his previous life to start new with the book people. Montag can no longer sit still and ignore the problems; although the war destroys the city itself, he knows that he can change the world by influencing the survivors elsewhere. Instead of giving in, Montag has lit the flame of his own individuality, and now can spread that flame, "burning bright," among others. His own life has changed forever, and he can never return to the mental ignorance -- and with that, comfort -- of his old life.