Halloween costumes for three can be easy and fun to put together. Classic Halloween costumes for groups of three include characters from children's books and fairy tales, such as: Three little pigs, three little kittens, Three Blind Mice, Mad Hatter, March Hare and Dormouse. Most recently, the three stars of the Harry potter books have become a popular costume for a group of three. Dressing as Harry, Ron and Hermione is very simple. The three Musketeers is also one of the most famous Halloween costume. These fabled swordsmen have been the subjects of films, plays and comic books and have shown up in a variety of guises.