Suunto Ambit3 Peakcan take both elevation gain and loss into account when measuring distance. This gives you a more realistic distance measurement when you are exercising in hilly or mountainous terrain. In this scenario, 3D distance is greater than 2D distance.
If you are using 3rd-party software to analyze your tracks, check how the track distance is calculated. If the calculation is using 2D distance, you may not want to activate 3D distance in your watch.
Otherwise, if you are frequently exercising in hilly or mountainous terrain, you should activate 3D distance.
To activate 3D distance: