First, do you have a successful connection between Easel and S3?
If you in Easel click Carve (top right) and using the jog panel, do the S3 move?
If yes then measure the actual travel vs commanded travel for each axis.
You can edit the GRBL settings by opening Machine Inspector (CTRL+SHIFT+D on PC) and in the Console window write for instance $101=50 and press Enter. This change the step per mm value for the Y axis to 50. A reset of the controller may be required for it to take effect.
If commanded value was 100mm and the actual travel was 50mm then the formulae for the new value is:
(Current step/mm value * commanded travel ) / actual travel
Edit this new value as mentioned above.