Superman III: Supergirl

Superman III: Supergirl was going to be the original version of Superman III starring Christopher Reeve and this version was supposed to be extended to a smaller scale unlike the previous films featuring the villains Brainiac and Mister Mxyzptlk with an appearance by Supergirl. The original outline featured a father–daughter relationship between Brainiac and Supergirl, even though the two are cousins in the comics. Warner Bros. opposed the outline and made their own Superman III film.

Plot
Superman does battle against Brainiac and Mister Mxyzptlk. Supergirl was going to appear as a guest character in the series.

Why It Was Cancelled

 * 1) The cancellation reasons are unknown.

Result

 * This film became Superman III.
 * TriStar Pictures released Supergirl a year later in 1984.