These endings are sometimes seen on the ends of people's names. Each of them have some type of meaning that is added to the name.

chan: This ending indicates affection and is more commonly used when referring to girls.

kun: This ending is usually used for boys, but there are cases where it's put on the end of a girl's name. It basically indicates that the person is a young boy. However, a boss (usually business) might call people who work for him with this ending added to their last name. (e.g. Aoyama-kun)

san: This ending indicates respect. It's probably more commonly used when referring to an adult.