If model 3 is going to be mass produced, It will probably be tough to offer free charging for life to so many vehicles.
Only a possible build up of Solar Powered Superchargers prior to the Model 3 release might help offset the huge wave of cars needing the option for free.
I would suggest Tesla charge for Supercharging a model 3. There will be no need to upgrade the existing superchargers with payment options. The "Tesla way" would be to only allow payment via the comfort of your touch screen in your car.
The Model 3 would detect when a supercharger is connected to the vehicle similar to when a Model S or X detects supercharger connection...bypassing the onboard chargers.
Next a payment window would pop up on the Model 3 screen asking the owner to approve charges to their Tesla account (credit card, paypal) or perhaps the ability to enter your credit card number of choice on the screen.
Once the driver approves charges to their account the vehicle would start charging.
Supercharges have been popping up like crazy but it seems like they will be used to capacity with Model S and X.
Perhaps it will not be possible to accommodate Model 3 Supercharging with 100s of thousands of cars on the road.
If it does become possible, the above payment option might be the way to help finance the buildup of more superchargers.