Ahab is the captain of the Pequod, the ship in Herman Melville's Moby Dick, who is obsessed with catching the eponymous whale.
Falstaff is one of Shakespeare's greatest creations; he appears in Henry IV Parts 1 &2 and in The Merry Wives of Windsor. He is a comic character, a vain coward who accompanies Prince Henry until the latter becomes king, and casts him off.
Mrs Gamp is a character from Charles Dickens' Martin Chuzzlewit. She is a nurse and a midwife, an alcoholic who takes advantage of her patients to procure more alcohol for herself. Her character was so popular that black umbrellas, like the old one she habitually carried, became known as Gamps.