In theory, all 3.
But I wouldn't be surprised if their service to supercharger is SO low that they could well be going to license their service centers to another brand.
Imagine if, overnight, Model S/X/3 owners could bring their car to their Mercedez / Porsche / Leaf (stop laughing) dealers for service?
And those brands, in return, can all use Tesla Supercharger network?
Nissan would be one of the few companies with service networks that already have EV knowledge.
Chevrolet would fit into this category too, and Chevy is doing a MUCH better job of servicing a fleet of deployed EVs nearly as large as Tesla's in the US at least - GM's tax credit phaseout started only 3 months later than Tesla's.
Porsche and Audi are also possibilities but they have nowhere near the numbers of Tesla or Chevy
The much maligned dealership model also means that a dealership isn't going to be punished THAT harshly for servicing a competitor's vehicles. The Chevy dealership I bought my Bolt from handles the more minor service items of the BMW dealership next door.
The big problem is - the existing Supercharger network isn't compatible with existing deployed vehicles from anyone else, and unless Tesla offers an SC charger to CCS adapter, the chances of GM corporate changing their entire fleet to use a proprietary standard (SC) instead of CCS are slim to none.