I would have to believe that a Tesla Service Center will service any Tesla: Roadster, S, X, 3, plus whatever comes down the pike later on. You need to make it easy on the customer, and to have certain models only serviced at location Z, while other models must be serviced 45 miles away at location Q is not only too confusing, but also insulting to the customer. (At least in the old days, any GM dealer could service any GM brand.) Sales will be handled similarly in those states that permit Tesla stores.
There are just not enough service centers around. Many owners today are hundreds of miles from one, and likely will continue to be hundreds of miles away when the Model 3 makes its debut. The costs to staff and maintain a service center will be amortized better the more cars that are in the service center vicinity that will need servicing, whether warranty work or general fee-for-service repairs. Tesla may have technicians that specialize in the Model 3, or the S/X, but the service centers will service all models.
As far as Supercharging is concerned, I believe that Supercharging will be "standard" in a package that includes fancier interiors and other bells and whistles which obviously would drive up this "base" price by many thousands of dollars. It will be an option for a separate fee much like it was with the 60s. Finally, for those who cannot afford or do not want "on-demand" Supercharger access, the Tesla website will have a 10/30/60/90-day access pass that can be purchased via credit card for unlimited use for that selected period. Their software will automatically enable supercharging on the date chosen by the customer. The pricing will be on the high side, but still much cheaper than gasoline. This will enable the family vacation or the occasional trips to visit relatives that are 200-400 miles away.