Why? I'm not seeing three worlds when one could serve just as well. Some sort of created wasteland isolated either naturally or artificially works just as fine for your last area. The setting itself determines what exactly caused it (zombie virus, nuclear weapons, magical blight, climate catastrophe, island floating too high above the clouds to be adequately sheltered), but isolation and a lack of resources will lead to things such as abandoned stuff, banditry, cannibalism, tech cobbled together from whatever's at hand, people doing strange things, people trying to secure their own settlements, people trying to escape but failing to cross whatever the particular barrier is, etc. You don't need a completely new world, you just need an absolutely terrible place to live, a reason for people to be there, and a reason why those people can't leave easily or be rescued easily.