The third season has indeed been cancelled; instead it was replaced with a movie that has just recently come out. This is described by this ANN news article.
Sora no Otoshimono Final: Eternal My Maser
Quick Plot Summary / Overview: The scene opens with the Zeus' destruction above Synapse, Tomoki calls for Nymph to return immediately; unfortunately Ikaros responds that Nymph isn't coming. Tomoki commands Ikaros to take him straight to Synapse as the Earth's surface deliberately vanishes. While flying towards Synapse, Ikaros tells Tomoki a story how long ago due to malfunction she was once ordered by a "boy" to attack Synapse out of desperation. Remarking a dying Sugata, Ikaros' body begins burning away because of a device installed within her whenever she approaches Synapse without permission. Having second thoughts, Tomoki orders Ikaros to go back but cannot since there is nowhere else to go to.