Piteous is something which elicits a strong sense of sympathy or compassion: “One is filled with piteous emotions on seeing hungry and naked children begging at public places.”
Pitiable means wretched, paltry, lamentable: “As an actor, the son could not match the success of his father as he had a pitiable lack of talent.”
Pitiful means touching, or exciting compassion: “The old man shivering in the cold and rain was pitiful sight.”
