It seems the last few months, we’ve been hearing more news about Alien 5 than AvP3. There’s a news report on IMDB where Sigourney Weaver has mentioned Alien 5:
And the star isn’t ruling out a return to the big screen for another Alien film – insisting no one else could take on the role. She says, “I would feel very weird if they brought her (Ripley) back and cast someone else.” But the actress insists she will only take on projects if the plotline works: “I don’t care whether the budget is big or small, or if it’s a lead role or not. If the story’s good, I’m there.”
With James Cameron tied up with Avatar and Ridley Scott directing a couple of movies, I don’t think we’ll be getting Alien 5 for a long time.