The main setting of the book is where the characters of the book are usually at. If they are at the park more than they are at home, then the main setting would be the park.
THe three minor characters are the characters that add the most to the story. If the book is in first person, then one of the main characters is the person talking. But the three people that are in the story most and talked about most are the three main characters.
Hope this helps :)
