Headliner install

As far as which of the three holes to use you should have marked them while removing
original headliner.
Since you have no marks you need to do a dry run fitting of the bows without the cloth to determine the fit of the bows to your individual car.
The 3 positions are there to adjust for the differences or factory tolerances from car to car.
There needs to be a slight arc in the bow creating some spring pressure when installed. It should be able to swing front to rear some without dragging on roof skin. This allows free movement to stretch the fabric without it hanging up on car body. Think suspended and only fixed mounting around the perimiter.