Alien 3 and Alien Resurrection?

Ok, I've seen all the Alien movies with Sigourney Weaver but Ive seen them all at different times and out of order so I am still a bit confused. Can you clearify three things for me?? First, If Ripley died in the 3 one, how come she it still in the 4th(Alien Ressurection)? And second, in Alien Ressurection, there is this Alien that is half Alien Half Human(hybrid) I think, How did that happen?? Im so confused. And one more thing, How come in the 4th, Ripley is partially an alien too?