The first two are forks that fundamentally did not change all that much, the latter aims to create a "true" opensource alternative of VSCode (including the ecosystem, where VSCodium falls flat) to serve as a common base for (and by) a few industry giants to build their next-gen of IDEs on top of (an "IDE framework", per their description).

As for the first two, last I checked openvscode-server was just enough to host VSCode for the browser, code-server had a few extras, like hosting at a subpath.