The original CCS standard did not allow the charge speed that Tesla wanted, so IMHO, Tesla decided to build their own. THat also kept the 'riff raff' from trying to tie up the charging network Tesla spent a lot of money building. There was talk about letting others utilize their standard and share the costs, but that never happened. Don't know what the financial impacts may have been for that to have happened. The digital protocol used by Tesla would need, not only a new plug or adapter for a CCS unit, but a smart computer that could negotiate the required information for charging to start and continue properly. Since use of a supercharger can be either free or paid, somehow, the protocol must identify the vehicle to ensure they are authorized to use it...none of the CCS equipped vehicles have that capability nor is the plug the same.
The newest CCS standards now allow up to 350Kw/hr charge rates, more than the original Tesla, and far more than an i3 can handle. Tesla has been updating their hardware to allow faster charging speeds, and to be honest, there aren't all that many CCS units that can handle those high rates, either.
At least in the USA, Tesla Superchargers are off limits to anything except Tesla vehicles, both from a compatibility and functional issue.