One of the most interesting things about these films is how they shifted over time. They all have some horror elements, featuring blood and gore and terrible monsters, but it shifted away pretty decidedly from straight horror around the second film, and the third, Army of Darkness, drops it completely. The first is a B-horror, but a very good one. Blood spurts and monsters growl and there is a scene with tree rape but overall it is a good horror film with a very solid and atmospheric first fifteen minutes and a sometimes funny but often frightening latter two acts.
Evil Dead II is where the series really shines though, brilliantly combining horror and comedy, often jumping between them very quickly, within seconds of one another. The film also often goes from moments of shouting blood to immense silence, waiting for demons to come through any of the openings into the cabin, as they frequently do. The first was the quintessential cabin horror movie, lovingly mocked by the recent title The Cabin in the Woods. The second takes the concept and throws it on its head long before the meta concept had become so accepted. It shows what was a normal horror movie and laughs at it, literally in the case of one scene were the entire house begins to laugh after enormous geysers of blood had shot out from the wall.
That is the essence of the film, violence at a contrast to utter insanity and slapstick. Several of the actions in the film are taken nearly directly from the Three Stooges, though adding a good dosage of blood. Essentially the film is to normal horror what Shaun of the Dead was to zombie horror. It delights in this, with the main character, Ash, putting on just the right expressions screaming and shouting but looking defiant at precisely the right moments. The film is not a deep examination of why horror movies are popular, nor does it try to look deep into the human condition and explain why we do all that we do. It is simply a great B-Horror-Comedy thing that rides the line and manages to overcome the pitfalls of both genres.
Army of Darkness takes the ridiculous parts from the previous film and multiplies them by ten. Finally having a budget Raimi is able to direct a final fight involving knights and zombies and cars and an infinite ammo shotgun with one liners spouted after every kill. This is what the previous films were leading up to, the patent ridiculous overloading any sense of disbelief the audience had and going straight to awesome. That is the goal of any sort of action comedy, and it does it brilliantly, again highlighting the ridiculous of the concept and embracing it like any good B-movie.
Overall the series shows a sense of progression that is rarely seen in films. It's not a forced change like the sequels to The Land Before Time or the gradual degradation of a villain like the Nightmare on Elm Street movies, it is simply a forward progression based off the conventions of the genre and an attempt to subvert the expectations of the audience. There are few directors who could pull this off, as it requires an understanding of audiences and genres that simply is not had by many directors. Tarantino was able to do it with Kill Bill part 1 and 2, with all the violence in the first and talking int he second but that was a very rare case and only extended to two films, as well as not exactly changing genres, just changing focus.
Of course going from splatter horror to comedy could also be thought of as just changing focus. Most horror can become inadvertent comedy with just a little change, like to the music, or quantity or color of blood. Given how accustomed society has become to violence it only makes sense to combine the two, it is more of a natural extension than anything else, as shown by a recent upward swing in horror comedies. Overall the Evil Dead franchise has stood the test of time and will continue to be a cult movie for decades to come.