The 2021 Model 3 has no major changes. Tesla, which operates more like its Silicon Valley brethren than a traditional automaker, eschews model years for rolling changes. As developments arise, they go into production. Over-the-air software update beams these updates direct to your car, so you don't need to upgrade to the latest model for the latest features.
The latest changes to the Model 3, however, are more than software: the 2021s get larger, slightly more efficient batteries that result in another 13 to 31 miles of range depending on trim. All versions also feature some cosmetic enhancement inside and out. Rounding out the changes is a redesigned center console that can now hold and wirelessly charge two devices.