1. If the second one was great and better than the first (i.e. The Dark Knight, The Empire Strikes Back), the third one will always be hated or thrashed by the public (i.e. The Dark Knight Rises, Return of the Jedi).
2. If the second film was considered weaker than the first one (ie. Indiana Jones and the Temple of Doom, The Bourne Supremacy), the third will be hailed as a being as great as the first and a nice ending, with some complaints (ie. The Last Crusade, The Bourne Ultimatum).
3. If the series first two films are critically acclaimed across the board without a hint of hatred (ie. Toy Story Trilogy, Lord of the Rings Trilogy), the third will be considered the perfect closure to the the series.