If you're a fan of LEGION the novel, as I am, you won't like it.
If you're a horror movie fan who hasn't read Blatty's novels then have at it.
Also, why was the Regan McNeil timeline retconned to 1975? This movie is set in 1990, they refer to it as being 15 years later after Karras' tumble down the steps, and the date of death on Karras' grave marker reads 1975.
Blatty (RIP) was probably going for a "rounded" date - 15 years later seemed to work more smoothly in his mind and 1975 is an easy round number. Why he did this I don't know. I think he should have stayed with the original dating of the first story, which was 12 years after Damien's death; this resonates - in a way - with Merrin having performed the MacNeil exorcism 12 years after he performed the African exorcism.
FWITW, I am a fan of both the novel and the film. The film took liberties - partially of Blatty's own creation, some forced on him by Morgan Creek - but I think they worked in view of the ending that resulted. One of my few dissatisfactions with Legion is that it seems to have been written in a hurry, which resulted in too many loose ends. Its climax is really a let-down, if not an anti-climax, where the Gemini simply gives up on his killing spree when he realizes that his evil father has dropped dead, which the Gemini himself then does. I find this unsatisfactory for several reasons.
1. The Gemini may have given up, but this does not relate very well to Blatty's theme of the vengeful demon manipulating the serial killer. That is, why would the demon give up just because the Gemini lost heart? Wouldn't the demon, having found a perfect vehicle for demonic work, continue to use this prize possession (!) to further suffering and chaos in the world?
Granted, the demon did get revenge on everyone directly involved in the original case, except Kinderman (a close call, however, for the detective) - but that doesn't mean that the demon lost heart. On the contrary, the demon ought to have gone right ahead with its work (as it says in the film, it has much more work for its "son, the Gemini" to do). The film handled this aspect of the Gemini's demise much more sharply and definitively than did the novel (even though some of the exorcistic sfx were over the top).
2. Adding Karras as Karras back into the story - not just his resuscitated body as in Legion - "ups the ante" and gives Kinderman and Morning a real, poignant reason to involve themselves - Damien Karras himself is present in Cell 11, and the courageous priest is now himself in need of rescue. This is a far more gripping scenario than Legion's presentation of Karras' empty husk, animated by the Gemini. There are now two enemies to defeat: the vengeful demon and the dybbuk Gemini. And there is a new victim to liberate: Karras himself. These new elements make for a more involving story line.
3. Reiterating: Legion leaves a lot of loose ends, e.g., was Dr. Amfortas' great love, Annie, really carrying on an affair with the loathsome Dr. Temple? Was Amfortas' Doppelganger correct that Amfortas actually, willfully, acted in the Gemini crime spree, or was the Doppelganger just tormenting Amfortas? Had Temple really been talking to Vennamun about the earlier Gemini killings, or was this just a method that Kinderman used to frighten Temple? And where did Kinderman's emphysema go? It nearly crippled him in the original Exorcist novel. Why does Blatty let Vennamun off the damnation hook? He hints that Vennamun entered into Life Eternal in the company of his dead twin brother, who all along had been trying to persuade Vennamun to knock off the killing spree (as if he could, even if he'd wanted to - after all, he was under the demon's command). And why does Blatty show mercy to Vennamun but not to Amfortas, who at least had the "excuses" of chronic depression and a brain tumor to take the edge off the guilt of his criminal involvement in the Gemini's crime spree? ... and so on.
By omitting these characters, events, and the loose ends associated with them, Blatty's Exorcist III screenplay provides a much clearer. sharply defined narrative than does his Legion novel.