“Both the Model 3 and Model Y included Tesla's Full Self Driving Computer 3.0 from day one.”
Wrong. Hardware 3.0 was introduced mid production of the model 3. The first model 3 was produced in July 2017. Hardware 3.0 was added in March 2019. So there are a lot of Model 3 cars for which an upgrade is necessary parallel tot Model X and S to enable FSD.