There are actually two Alien3 movies due to different story elements.

Details with minimal spoiler:

[..] Instead an extended cut called the “Assembly Cut” was created based on his editing room notes and released on the 2003 Alien Quadrilogy box set, overseen by producer Charles de Lauzirika. This version of the film runs 144 minutes, and includes over 30 minutes of extended, alternate and previously deleted scenes and alternative key plot elements. One example of the plot differences is that in the theatrical version, the Alien bursts out of a dog, while in the Assembly Cut the Alien bursts out of a dead ox…