Even if it contains 3 parts, it was originally released as a single theatrical movie. So, it should be listed as a single movie.

@superboy97 said:

Even if it contains 3 parts, it was originally released as a single theatrical movie. So, it should be listed as a single movie.

Someone added them still last month:

• https://www.themoviedb.org/movie/1475828-psycho-pass-3-first-inspector-rainyday-and
• https://www.themoviedb.org/movie/1475819-psycho-pass-3-first-inspector-ziggurat-capture-part-1
• https://www.themoviedb.org/movie/1475826-psycho-pass-3-first-inspector-ziggurat-capture-part-2