Cross compile all three, as with any other project.
The number of subproject names or your perception of the organization does not change anything in that regard (except maybe complexity of organizing and executing that transition).
I bet that, if you take a closer look, you will find more "languages" already, like bash, make etc.
Edit: "Requires old versions" equally isn't an issue if the new code does not require new features which is ensured by being able to compile it at all. The only important part is to ensure that old versions in the complete cycle can be used to ensure that there isn't a rollout order dependency which would by an issue for practical reasons.