CoreOS is an operating system designed to run docker containers. You would use it as lighter weight replacement for something like Ubuntu as the base os that Docker runs on. When you install CoreOS you basically just get a kernel, Docker and etcd plus a minimal number of other processes. Etcd is designed as a way to allow a distributed set of containers to self-configure.
Flynn is an open source project to build a "platform as a service" platform on top of Docker. In theory Flynn could run on top of CoreOS.
Flynn is an open source project to build a "platform as a service" platform on top of Docker. In theory Flynn could run on top of CoreOS.
Docker is the core of everything.