The key difference is the FreeStyle Libre 3 sensor automatically sends real-time glucose readings* to the FreeStyle Libre 3 reader without needing to scan once the FreeStyle Libre 3 sensor is active. When using the FreeStyle Libre 2 sensor with the FreeStyle Libre 2 reader you are required to scan before you can receive glucose readings.
* 60-minute warm-up required when starting the sensor.