Explain how the concepts of rationalism, idealism, and humanism are shown through the words or actions of 3 characters in Antigone?
The main problem with this question is that humanism and rationalism were philosophical movements that developed almost 2,000 years after Sophocles (497-406) wrote Antigone. Although we can interpret elements of the play in terms of the moral view points of these philosophical movements, the characters in the play do not "show" anything about these movements, as there is no way the author could have been acquainted with them. Idealism is associated with Plato, who was closer in time to Sophocles, but did not publish his first dialogues until after the death of Socrates in 399 BC; thus although Sophocles may have influenced Plato, Plato could not have influenced Sophocles.
If you are using idealism in a philosophical sense, it refers to the belief that only ideas are real, and the phenomena are merely inferior physical manifestations of those ideas. Antigone and Creon both hold somewhat to this family of beliefs in the sense that both Antigone's belief in the authority of the gods and Creon's belief in the rule of law represent a belief that judgments should be made in the realm of reason or abstraction (the ideal rather than real worlds) rather than empirical fact, although Creon both characters do accept some empirical evidence at various points in the play.
For Humanism, one of our surviving Renaissance Latin adaptations of Antigone actually sees both Ismene and Creon as wrong in their pridefulness, and praises Ismene as a model of submission to a lawful ruler.
Rationalism is the belief that true knowledge comes from reason rather than observation or religious intuition and that the power of reason alone can enable us to understand the world. Creon is the character who is closest to holding this point of view.