Beyond the Sky

Planes 3 was a planned sequel to Planes: Fire & Rescue that was set to be the third installment in the Planes film trilogy, a spin-off to the Cars franchise by Pixar. It was originally going to be released on April 12, 2019, but was removed from the release schedule following the shutdown of DisneyToon Studios.

Plot
The film was planned to explore the future of aviation in outer space, rest of the plot is unknown.

Why It Was Cancelled

 * 1) DisneyToon Studios was shut down on June 28, 2018 (mentioned earlier).
 * 2) The first Planes film received generally unfavorable reviews from critics and audiences.

Result

 * The film's development was kept unfinished and was permanently cancelled.