The Master Argument due to Diodorus Cronos claims that nothing is possible that neither is true nor will be true and that therefore every (present) possibility must be realized at a present or future time. Unfortunately, it leads to logical determinism. In this paper, based on Prior's insight, a three-valued approach to the Master Argument is presented by developing a three-valued modal tense logic with a Kripke semantics. We also discuss philosophical and logical issues in connection with other approaches.