I'll have a go
The M3 (European model) car only comes with a CCS socket. CCS is the new international standard, which didn't exist when MS was conceived ...
Tesla has fitted dual-cables on some of the Stalls at Supercharger sites - already most of the sites in Europe, and about 50% of those in UK (even though no RHD M3s delivered yet) have this modification. So you can continue to use those sites, and indeed those stalls - just use the old connector, rather than the new one. (They are radically different shape, so no chance of accidentally "putting Petrol into a Diesel" ...)
There is talk of a CCS adaptor, which would allow any existing MS / MX to plug into a CCS Charger. That would allow you to charge at any CCS stall, there are hundred all across the UK, but none (or literally "only one or two") in the UK are Rapid; there is a big expansion of Rapid CCS chargers across Europe, so only a matter of time until they are widespread, but of course Tesla is also rolling out more Supercharger sites too ...
So once the CCS adaptor is available you will be able to charge at any CCS site, including if Tesla in future make any CCS-only Supercharger sites.
An M3 will therefore be able to charge at Supercharger (at a stall with the modified cable) or any CCS site. The SatNav on M3 only shows Supercharger sites which have modified stalls, which i think is a nice touch
Whereas other CCS-capable brands of EV cannot charge at Supercharger site
3rd party CCS sites are like any other site ... often broken, often only one or two stalls so may well be occupied and you have to wait, despite being an "international standard" there are plenty of stories of Car-Brand-A not working with CCS-Brand-B, so its not all plain sailing ... yet.