Unfortunatly I have to correct you there.
Pillboxes were constructed on stop lines to stop advances of enermy forces. You may have just noticed three, they would span in lines following rivers and railways inland.
Some were just constructed in strategic areas to cover bridges and areas of importance.
This is why when you enter a stop line pillbox it has a number for exmple "A274" and they would run in sequence. From memory this is because of the old scale 1" maps could have multiples in close proxy of each other,to enable troops to man the correct one(locating at night etc)
Coastal pillboxes do complement each other with arcs of line of sight as they have one common frontage, but inland it was to stop advances and slow incoming forces.
But no link to groupings of three
hope that helps
Paul