We've started the public beta for the experimental client ports to Linux and Mac. They are available as a complementary service to anyone who owns Arma 3 for Windows. Please check out the overview page for more details, limitations and a F.A.Q.
You can access the beta ports by installing Arma 3 while Steam is running on a port platform (main branch). Please report issues on the Feedback Tracker, and leave us your feedback on the forums. We're very interested to learn how the ports work on your hardware, drivers and OS versions, bad or good!