No need for that. The blog post says "For Teslas ordered after January 1, 2017", if they were going to exclude the Model 3 it would specify that it was only for the Model S and X.Elon mentioned that those who put down the early reservations on the M3 were going to get something special. The same deal (1000 miles free/year SC) would be my vote for the "something special".
They can now even sell a chademo adapter (tied to a Tesla account) to allow non-Tesla EVs to charge. While making money doing it. This, perhaps more than anything else Tesla does, will really accelerate the transition to sustainable transportation.
Given the way things work now where the car tells the SC that it is ok to charge it, it would require more than an adapter to let other makes charge at the SC. The other manufacturers would need to add some way for their cars to talk to the SC.