As different tonally as they are (one is hide and seek, the other tag you're it) Alien and Aliens have always felt like two sides to the same coin to me, they compare and contrast really well, and a lot of Aliens feels like a natural extrapolation from Alien. They both present a more or less indifferent universe where the characters just happen to stumble upon a particularly nasty corner of space, and are able to take charge and survive if they're lucky/smart enough.
3 is different in that right from the start it's everything is awful and we're all f**ked - the universe is trying to put you down no matter what. People often say it's more in line with Scott's film, perhaps on a surface level detail it is, but I can't shake off its overwhelmingly/obnoxiously dreary tone it never quit earned. (That's not to say I don't like Ripley's final sacrifice and all that, but I would never say the film is more than the sum of its parts).
And then you have Res which is like Aliens on absinthe.
