When Model-S came out ... 7 years ago I think? ... there wasn't a suitable charging standard so Tesla created their own. The world could have adopted that but ... Politics
CCS is the new standard for Rapid charging (there are others like CHAdeMO but they look unlikely to win the VHS-BetaMax race). CCS is a committee-compromise-thing - the Plug is huge, and therefore you have to have a Big Flap to cover it, which is far more likely to get broken off and so on. Anyway ... it-is-what-it-is.
Tesla decided to adopt that CCS standard for (non-USA) Model-3 rapid charging .... M3 can therefore charge at (suitably converted) Supercharger ... or any 3rd party site with CCS chargers (of which there are huge numbers ... but currently they are quite slow, and many are unreliable). Huge "Really Fast CCS Charger" rollout "coming really soon now" ... it is definitely happening, but not very quickly, and Supercharger Network size is increasing too ...
Indeed, there are CCS chargers which are faster than Supercharger (until next generation Supercharger gets here). So kinda convenient for M3 owners to have the choice. Notwithstanding that 3rd party charging in UK is pretty much universally dreadful - broken stalls, slow stalls, very few stalls at each site, cars with very slow charging speeds (so if there is one occupying the pump you want you will have a long wait) and so on.
Quite a few stories of incompatibility between THIS brand of car and THAT 3rd party CCS charger site ... none of those problems with Supercharger because end-to-end it is Tesla and totally under their control.
But ... "it is the future"