Exorcist 3 was a mildly interesting failure--the book Legion and the film adaptation shared the same weakness--mainly that they are way too talky. Anything in Exorcist 3 that a horror fan would actually want to see happens off-screen. If you want to see a movie in which a series of murders is described to you through dialogue then Exorcist 3 is definitely for you.
William Peter Blatty actually came up with a pretty cool story--basically there are a series of grisly (or so we are told) murders and the trail eventually leads to the psych ward of a hospital which has a patient in solitary who is the splitting image of deceased Father Karras from The Exorcist. He also was admitted to the hospital around the same time that Karras. died. The mental patient knows details about the murders that only the killer could know but he's kept under strict lock-down, so how can he be the murderer?
While the hospital patient is supposed to be the spitting image of Father Karras (Jason Miller) anytime there's dialogue he's played instead by Brad Dourif, which might seem odd but it turns out to be the best choice William Peter Blatty made. In Dourif's mouth the talky dialogue actually comes alive and the scenes become electrifying. The rest of the time the movie is generally a bit dull and way too restrained. It's a shame William Friedkin didn't direct this--restraint was never part of his cinematic vocabulary--it's likely he would have hit this one out of the park.
I haven't seen the theatrical version since it played in theaters so I can't really compare it to the fan edit. I can say that Legion was much more entertaining than I remembered Exorcist 3 being but the new ending is pretty abrupt and unsatisfying--it's very possible that this ending is an improvement but I don't even remember the original ending so I definitely couldn't say.
I would recommend this fan edit of Exorcist 3 but viewers shouldn't expect a lost classic because the original movie really wasn't very good.