How can I associate 3 authors with just one book?

I do not know if you're familiar with Collections and Generics , but this is a way to solve the problem.

Instead of its Livro class, it has an attribute of type Autor , you can have a List<Autor> that represents a list of authors of the respective book.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Livro {

    List<Autor> listaDeAutores = new ArrayList<>();
    // Outros atributos do livro...

    Livro(Autor autor){
        listaDeAutores.add(autor);
    }

    Livro (Autor...autores){
        listaDeAutores.addAll(Arrays.asList(autores));
    }

    // Métodos...
}

That way you can create a new book containing an author. For example:

Autor autor = // ...
Livro livro = new Livro(autor);

Or, with several authors. For example:

Autor autor = // ...
Autor segundoAutor = // ...
Autor terceiroAutor = // ...

Livro livro = new Livro(autor, segundoAutor, terceiroAutor);

To display the list of authors you can retrieve the list contained in the listaDeAutores attribute and loop through it:

List<Autor> autoresDoLivro = livro.listaDeAutores;

for(Autor autor : autoresDoLivro){
   System.out.println("Nome: " + autor.nome);
}

Is not related to what was asked, but consider encapsulating the attributes and methods of your objects.
This question has some explanations.

If I understood correctly you could do a construtror in the Book class that gets 3 arguments where each would be an author. That's if you want to always associate 1 author or 3 authors with a book. If you prefer to make this association dynamically (ranging from 1 to x authors per book), I believe a solution would be to use array of x positions where each position holds an author. Already only pass this array as argument in the constructor.