Fritzner explains it as <Beslægtet i tredje Led> 'related in
the third degree'. As degrees of consanguinity are normally
reckoned, second cousins are within the sixth degree, not
the third: each is three generations removed from the common
ancestor, so to get from one to the other through the family
tree requires six steps, three up and three down.
Apparently the <þrí-> here refers rather to the number of
generations separating the cousins from the common ancestor.