I think Takuro Fukuda is a writer who gets a bad wrap in the Rider fanbase (serving as head writer on the often panned Kamen Rider Ghost, as well as mixed-reviewed Kamen Rider Saber, despite writing only a handful of episodes of the latter himself). However, this movie is a clear example of Fukuda's ability to write amazing, powerful stories. Then, coupled with the amazing, methodical, and impactful directing of Kazuya Kamihoriuchi, we are gifted a masterpiece.
Trio of Deep Sin is not an entirely original movie, I could point to elements it borrows from other Kamen Rider properties, but it's style and intent are wholly its own. Trio of Sin is a devastating yet beautiful gut punch, that personally left me in tears a number of times, and has become a film that I can't stop thinking about days after watching it
To be frank, I do not want to provide a single specific spoiler in this review, this is an incredible movie, and if you are a fan of Kamen Rider, or just *good movies* this is one you should definitely give a watch!
If you haven't seen the show this film serves as a sequel too, I'd say the film is still watchable, but you may be left scratching your head more than the film intended. If you have no intention of watching the show, but do want to watch this movie, I would highly recommend seeking out a summary of the show first.