Tesla Models S and Model X have free access to Tesla Superchargers but people who ordered the Tesla Model 3 who did not order the optional Supercharger package have to buy credits before they could hook up with any Supercharger.
Tesla Model 3 'Supercharger Credits' discovered on 'MyTesla' page
...someone offered their Referral Code, get a $1,000 credit on your own Tesla.
https://www.tesla.com/referral/matt327