The three great masters of short stories you mentioned are O Henry, Maurassant, and Hugo. Mark Twain, however, was an American. He was not one of the three great masters of short stories. His masterpieces include Tom Swift, Huck Finn, and Crusoe. Mark Twain's works were humorous and satirical, reflecting and criticizing the American society and culture at that time. He was one of the most important figures in the history of American literature and was also known as the father of modern American literature.