Hi
Keep in mind that Apple Watch can pair and sync with compatible gym equipment, by following the instructions here:
Use gym equipment with Apple Watch
If estimations for Indoor Run workouts were inaccurate on your Apple Watch Series 1, then I would certainly suggest resetting your calibration data and calibrating your Apple Watch Series 3 (GPS) afresh.
Whilst calibration will continue automatically on your new Apple Watch during future Outdoor Walk and Outdoor Run workouts (subject to your settings - see below), for more accurate estimations from the outset you may wish to follow the same procedure as for Series 1 and earlier models, completing some initial calibration activities at each of the speeds at which you typically walk and run.
Calibration teaches your Apple Watch how your arm movements relate to your stride lengths at different speeds when you are walking and/or running. It does this, during outdoor walk and/or outdoor run workouts that are recorded via the Workout app, by comparing data from the accelerometer with GPS (Location Services) data.
Before doing so, check your Location Services settings:
On your iPhone, go to: Settings > Privacy > Location Services:
- Check that Location Services (the main setting at the top) is turned on.
- In the list of apps beneath, check that Apple Watch Workout is set to While Using.
- Also enable options under System Services including Motion Calibration & Distance, Wi-Fi Networking and Mobile Network Search.
To then calibrate your Apple Watch - and to reset existing calibration data - follow the instructions here:
Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
Resetting your calibration data can be equally as helpful for Series 3 models and will not erase your Activity history.
For the best results, be sure to allow the arm on which you are wearing your Apple Watch to swing naturally during treadmill-based workouts and also during calibration.
Also keep in mind that, when estimating distances for indoor walks and runs via the built-in accelerometer, Apple Watch cannot recognise or adjust for inclines or declines on a treadmill.