I'm in the process of bulding a solution for managing all three OS'es. AD is not on the table because theres nothing to do with kerberos in our network and AD would be a "windows only" solution.
Why is AD a Windows only solution? Large corporations and startups use it to run tens of thousands of Macs and Linux machines in addition to Windows. In fact, I can't think of a single large company that does not use it. Its basically the core for many.
Linux can totally run in a AD domain with auth managed by AD. Client side SMB is also not bad. But you are excluding Kerberos for some unrelated reason, right?