So, you are right—the Model 3 does indeed have adaptive cruise control. When cruise control is enabled, your Tesla’s exterior cameras detect your on-road surroundings, then speed your vehicle up or slow it down to maintain a set following distance to the car in front of you. With Autopilot engaged, your Tesla will continue to accelerate or brake depending on the traffic around you until you disengage by manually braking or taking control of the vehicle.
Some older Model 3s might not have Autopilot installed, and therefore will not have adaptive cruise control. If that is the case, you can subscribe to get the Enhanced Autopilot package for $199 through the Tesla App or your Tesla account.