Naming them this way has them appear as three separate posters in my library. When I then go and fix the match, they are bundled together, but plex thinks that they are versions of the same movie.
Is there a step I am missing. Should the files be named something else?
I have gotten multipart movies to work for me. I think your issue is the “part” nomenclature. I have had success with naming them as “xxx - part1.mkv”, etc. I don’t think that “ptx” is the correct format.
You need to omit the space characters before and after the hyphen.
This is different from the way series episodes are named, where spaces are recommended around the hyphens.