Is the long range battery in a 3 100kWh rated?
No.
A quick Google search for "Tesla Model 3 battery size" indicates that it is 75kWh.
If true and I charge 50% is that 50kWh?
If it were a 100 kWh battery, yes. But it isn't. It's a 75 kWh battery. Therefore if you add 50% then you've only added 37.5 kWh.
Therefore at 21 cents per kWh it would cost $10.50?
If you added 50 kWh, then at $0.21 per kWh it would cost approximately $10.50.
However, if you add 37.5 kWh (50%), then at $0.21 per kWh it would cost approximately $7.88
Note that the amount you pay may be slightly higher than this. If I recall correctly, Tesla will charge you for the amount of electricity the Supercharger supplies, not the amount of electricity that your vehicle stores. Charging a battery is not 100% efficient. There will be some losses due to resistance in the cable from the supercharger to your vehicle. There will also be some losses in the vehicle equipment. I'd guess that the charging efficiency of supercharging is somewhere around 95%. So, if you add 37.5 kWh to the battery, you're probably using approximately 39.47 kWh (39.47 X 95% = 37.5) from the supercharger. At $0.21 per kWh that means you'd probably pay somewhere close to $8.29 instead of $7.88
I hope this has been helpful.