A master of death does not fear death. Mastering the Deathly Hallows does not make one "invincible." It would take someone who accepts death to master the three objects. Harry defeats Voldemort, because Harry's capacity to love and trust his friends is more powerful than Voldemort's hate.
Harry used the Resurrection Stone to help him face death. Harry used the Cloak of Invisibility he had inherited to protect his friends and defeat Voldemort. Dumbledore kept the Elder Wand to prevent it being used for evil purposes. Harry became its master when he captured Draco Malfoy's wand who had unknowingly become the master of the Elder Wand when he had disarmed a weakened Dumbledore. After repairing his old phoenix feather wand with it, Harry returned the Elder Wand to Dumbledore's tomb because it would only attract trouble.