In chapters 1-3, what was Calpurnia's fault?
According to Scout, who is the narrator of the story, Calpurnia has many faults. It is only later in the story that Scout comes to appreciate Calpurnia, who is the family's cook. In the first chapter of the book, Scout introduces Calpurnia. She describes the woman as being nearsighted and having to squint all the time because of it. Scout describes Calpurnia's hands as being "wide as a bed slat and twice as hard." This shows that Calpurnia is a disciplinarian around the Finch house, which naturally a child would dislike and consider to be a fault. Scout considers Calpurnia to be a bossy woman, and she describes their disagreements as "battles." The older woman usually wins these battles, much to Scout's displeasure. Calpurnia asks Scout "why [she] couldn’t behave as well as Jem," and she also calls her home when she does not want to come in. These are all things that Scout hates. In the second chapter, Scout blames Calpurnia for having her copy chapters from the Bible in neat penmanship. Scout's teacher, Miss Caroline, disapproves of this type of handwriting. In the third chapter, Walter Cunningham comes over for lunch and Scout is rude to him. Calpurnia confronts her about it, which Scout does not like. The woman gives Scout a smack on her bottom. Scout asks her father to fire Calpurnia, but he refuses. All of these things are considered faults of Calpurnia according to Scout.
I think that what you probably are asking about here is in Chapter 2. That is, if you are saying that something happened and it was her fault, that's where it is.
What happens is that Scout gets in trouble at school because she already knows how to read and the teacher does not think that is appropriate. The reason that it is Calpurnia's fault is that she is the one who taught Scout to read.
If you say "Calpurnia's fault" and you mean like what was wrong with her, Scout seems to think (Chapter 1) that Calpurnia is too strict.