Thor 3 will come around just prior to the third Avengers movie, which we anticipate will see Thanos as the main villain, so it will be about the right time for Thanos to start making himself known to the heroes. As we already know, the Tesseract - which had previously been in Thanos' possession - was taken to Asgard at the climax of the Avengers movie, so Thanos will need to retrieve it once more in order to wield the full power of the Infinity Gauntlet, as we all expect him to do in the third Avengers instalment. For that reason, he's going to need to be involved in the next Thor film. At the very least, he should send a lackey to Asgard and we should hear Thanos speaking for the first time when instructing him to do so or receiving the delivery from him (assuming he doesn't speak in the likes of this year's Guardians of the Galaxy).
