Model S had a separate stalk for TACC adjustments and TACC/Autosteer activation. The 3 does not have the extra stalk, so the right scroll ball is dedicated to speed/follow distance adjustments, and the shift stalk activates TACC/Autosteer.
When I test drove a new Model 3 (after 3.5 years of driving used Model Ses) I found it to be a simple, intuitive solution.