Not quite as good overall as Aliens, the second film in the series, but still a cut above Alien, in my opinion. Alien3 has a different flavor than its predecessors, and despite claims made for them, this one actually has a subtext. Unfortunately it also has flaws that knock a couple points off its rating.
In an unusual move for sequels (but, as far as I can tell, a move that most fans would prefer), Alien3, like Aliens, picks up the story right at the end of its parent film. Because of this, it is even more imperative that you watch the Alien series in chronological order. It may not be required with, say, Urban Legend and Urban Legends: Final Cut, but with the Alien series, you'll ruin the effect a bit otherwise.
Alien3 director David Fincher, like Aliens' James Cameron before him, opens his film with what amounts to the closing shot of the previous film. Ripley is in "suspended sleep" after having battled the "queen alien" on the terra-formed planet where the crew first encountered the beasties in Alien. But this time, as the white on black titles alternate with shots of Ripley's ship, we see an alien threat immediately. We never know how the alien face-hugger got on board exactly, but Ripley ends up being the only survivor out of three humans. The section of the ship they are in is emergency ejected after the ship's computers sense the alien and a fire is started, and the sub-ship crash-lands on a desolate planet that only houses a prison.
The prison is unusual in that it doesn't have many inmates, and the inmates that it does contain are no longer prisoners, exactly. They freely roam around with the staff and almost all espouse a common, religious-oriented philosophy that results in the prison more resembling a gritty commune. As you might guess, since this is an Alien film, Ripley and the sub-ship end up at the prison along with an alien menace, and the cast is left trying to figure out how to dispose of it.
Fincher and the scripters make a number of smart moves. They retain not only plot continuity but a stylistic continuity from the first two films. Alien3 is also dark, greasy and claustrophobic. The prison is also a lead smelting plant, so we have the best excuse of all three films for the industrial equipment and chains that make up part of the atmosphere. There are numerous narrow, low-ceilinged corridors, lots of airtight doors, big scary fans, etc. Fincher also decided to go back to having just one alien instead of the all-out assault of Aliens, and it enables lots of scenes similar to the first film in that they function more like the pursuit of a ghost through a haunted house.
(The next two paragraphs contain some spoilers, so please skip them if you haven't seen the film yet.) Like Cameron, Fincher also extends the alien mythology a bit, although it's not done with the explicitness of Cameron's "ant/termite colony theory," so we have to figure out the extensions on our own. For instance, we learn that the aliens can gestate in different life forms, and when they do, they take on some of the characteristics of those life forms. We also learn that the queen propagates in a way that's a bit different than the "worker/soldier aliens."
Most unusual, perhaps, are the religious overtones - from the literal beliefs of the prison population to the extent that real subtexts enter Alien3. One of the subtexts, although a shallow one, is that despite professed adherence to a credo, there are varying degrees of commitment among the prisoners. Some seem to be participating just to go along with the crowd. Another point is the temptation of evil (admittedly this is pretty straightforward, so it's not necessarily a subtext) that comes to a head most blatantly with the attempted rape of Ripley. But most interesting to me is that Ripley can be seen as the coming of the messiah, and the prisoner population comprises her disciples (only in this version most of the disciples are killed, unfortunately). At the end, she must be sacrificed for the continuance of hope for mankind. This subtext is made even more obvious given that the next film in the series is Alien Resurrection.
Despite these successes, and the many intriguing approaches taken by Fincher, and despite the tension of the action sequences - a tension that often matches the best scenes from the first two films - Alien3 has a couple problems, although only one major one. Many crucial scenes, including quite a few of the action sequences, are simply confusing. It's hard to know who to blame for this, but at least three or four times I was at a loss to know what was going on, exactly. Characters were running through corridors, opening and closing doors, crawling through shafts, etc., but I was often not sure which characters they were, where they were in the prison, or why they were taking specific actions. These scenes lose all sense of setting and plot - I had no sense of the overall scheme of the prison spatially and I wasn't sure what some of the threats or the proposed/attempted solutions to them were. One guess is that like Cube or Phantasm, say, there were only a couple corridor sets built, but unlike those films, the attempt to make them differentiated and coherent failed. It happens often enough for me to subtract points for it, but not often enough to ruin this otherwise fine film, and certainly not often enough for me to not recommend it.
Director Fincher disowned the film, and walked out on post-production.