The distances from groove centre to groove centre are:
1. (at the rear of the car) to 2. (centre): approx. 21 cm
2. to 3. (at the rear): approx. 19 cm
The free space between the individual rails is:
between 1st and 2nd : approx. 13 cm*
between 2nd and 3rd: approx. 9 cm*.
* where straps are attached approx. 2 cm narrower

The question is imprecise. Which grooves are meant? Maybe the distance between two bicycles?
Most of the information can be found here: https://www.thule.com/de-de...