What are three good examples from the novel Fahrenheit 451 that best fit the definition of science fiction?
I would agree with the answer above that lists taking place in the future, depicting a different society, and imagining technology we don't have are, in general, the most important attributes of science fiction. I would, however, also differentiate science fiction from fantasy by noting that in science fiction, the writer makes an attempt to use science that could plausibly exist: the laws of physics should apply. Science fiction generally steers clear of magic, unless it is "magic" that can be explained scientifically. Fahrenheit 451 fits this definition of science fiction quite well: for example, television screens the size of a wall of a house, while not available when Bradbury wrote the book, are physically possible, and we have come much closer to having them in our own society. Likewise, a mechanical dog armed with a poison dart is completely plausible in terms of our current understanding of science and technology.
Science fiction also tends to project into the future possible dire outcomes of problems we face today and so functions as social commentary. In other words, it uses the future to provide a critique of the contemporary world. Fahrenheit 451 does that well. It was written out of concern in the 1950s that television was replacing reading and that the content of television was low quality. It also critiqued technology, conformity and our increasing lack of contact with other people. These concerns are still with us today, so the novel continues to encourage us to take a step back and think about how we are living.
Finally, I would add that science fiction novels tend to (though it's not required) employ a good action-adventure story to keep the reader interested, with the futuristic technology playing an active, rather than a passive, role. In Fahrenheit 451, both the giant view screens in their hold on Mildred and the mechanical hound in its pursuit of Montag play an active role in the story: they are not simply backdrop. The novel also has an action adventure plot in which Montag rebels against his society, kills his boss and flees, with the state in hot pursuit. Again, this is not necessary to a science fiction novel, but nevertheless is often an attribute.
The novel contains several basic elements of science fiction, beginning with the setting.
1. The novel takes place in the future, the most basic element in science fiction.
2. Society in the novel is very different from our own. Montag's society is oppressive and totalitarian. The government keeps its citizens under control, primarily by keeping them ignorant and frightened. Individuality is condemned and individual rights are nonexistent. Books are burned to prevent independent thinking. The people lead miserable lives, largely unaware of their own suffering. Mildred's attempted suicide is a good example of this persistent misery.
3. The novel includes technology that is currently beyond our means. The primary example of this is the mechanical hound with its poisonous needle nose that is capable of tracking down traitors who dare to own books.