Actually... thats because they are different kinds of aliens. The alien in Alien 3 was born from a dog, Aliens absorb some of the gene traits from is host, there fore it was a dog. The Alien in Alien was different simply because the point of the movie was to show the Alien the least possible. Watch the directors commentary and you will see what I'm talking about. For all we know, it could have been the Alien in Aliens, but simply hidden in the shadows.
In Aliens, there are 2 kind of Aliens, the fleshy kind (fat, more humanly porportionate), which were men dressed in costumes, and the bony type.... see only a few times, which were pupets.
As for Alien:Ressurection.... bah... they are genetic freaks.
BTW.... AvP sounds like a real piece of garbage, plot wise. <_<