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i just got back form PA 3 and I think that there are different stories in each one (if u have noticed) that the whole area and characters are different

It's never confirmed that the mom was killed. She is seen being held up in the air and then tossed down the stairs. In part 2, I think it was part 2, the girls mentioned that they don't want to end up going crazy like mom.

Although there's obviously a lot of story missing between the events of part 3 and and 2. The mention of brain washing explains as to why the girls don't remember a thing about that time in their life. It is possible the mom survived, got brain washed to forget.

The writers of PA3 have answered what was going on at the end. Grandma is the leader of a coven, the same one that is mentioned earlier in the film. The coven is preparing a seance. They mention the step dad/boyfriend guy getting killed but not the mom.

I was thinking the same thing, I just saw it last night.

The best explanation I can think of is that something with the grandmother and her possibly brainwashing the girls then giving them to someone else were they forgot about Dennis and Julie.

I don't really know though, it can only be answered if they do a 4th film.

Also did you notice how these 3 movies have horse figurines in them? Especially the 3rd one.

maybe she has a step mom or someone who's a motherly figger to her and they didn't say in 1 and three she is the witch shes not dead