Write a story in which a maid, a child, and a policeman played an important role?It's an essay.

This is fairly wide open.  If the topic is to write a story in which a maid, a child, and a policeman play an important role, then I think that it belies a traditional essay, and is more open to interpretation.  You can find many examples where the three roles are represented in literature, and while they may not be explicitly mentioned in these terms they are present.  Rushdie's Midnight's Children is one such instances.  Servants are present, as well as a policeman as well as a child.  Mary Pereria is a maid/ servant who commits the critical action of switching the two children at birth, Shiva and Saleem.  Both children play vital roles in the work, especially the latter who is the narrator.  Finally, the former, becomes a police officer, and is charged with destroying the union of Midnight's Children.