Blocking Sets of Index Three
  In this note we will provide proofs for the various statements that have been made in the literature about blocking sets of index three. Our aim is to clarify what is known about the characterization of these sets. Specifically, we provide constructions for all R\'edei blocking sets in PG(2,q) of index three and explicitly determine all blocking sets of index three in PG(2,7). 
William E. Cherowitzo and Leanne D. Holder
math.CO math.AG