How does American Dream affect Gatsby, Myrtle, and Daisy ?
Gatsby attempts to achieve the American Dream, the idea that anyone can prosper with enough hard work and perseverance, but he ends up resorting to illegal means in order to acquire his fortune. He is a bootlegger -- someone who profits from the illegal production, sale, and distribution of alcohol during Prohibition. One who has earned their fortune by illegal means can hardly be said to have actually achieved the American Dream. Further, Gatsby's attempt to achieve the Dream by becoming rich and securing the affections of the woman he loves end up costing him his life.
Myrtle is married to a man who also tries to achieve the American Dream, and her husband's absolute failure to do so is yet another clue that the dream is actually an impossible one. Her husband, George, works hard and tries to get ahead legally, but he finds it essentially impossible. Myrtle attempts to achieve the dream in her own way -- by allowing her married lover, Tom Buchanan, to buy her expensive things that her husband cannot afford. However, Myrtle dies a terribly violent death because of her attempt to achieve the dream, as does her husband (and Gatsby as well!).
Daisy, ultimately, is the character on your list who has the least to do with the American Dream. Her own money, and her husband, Tom's, money, has been passed down in their families: it is "old money," money that has not been recently earned. Gatsby's money, on the other hand, is "new money," and new money is a great deal less valued than old money is. New money is what one might gain by achieving the American Dream (if such a thing were possible in this book). Therefore, Daisy really has nothing to do with the dream, except for the fact that once she finds out how Gatsby has made his money (and that he has not really achieved the dream because he's a criminal), she abandons him and returns to her husband. Daisy, who has no need to try to achieve the dream, ultimately prospers.