I have spent a lot of time figuring this out. All the articles, gists and whatnot were incomplete, complicated or non functional.
This should work out of the box, if I didn’t forget any github configuration, what might be necessary. For Android you just need to set up the secrets in the repository.
It builds all the platforms in parallel. It can also create pre-release builds.
I don’t own an apple computer(just a virtual macos), so for now I didn’t even try to make the build work for that. I plan to do it.