I'm sure it's structured such that the third-year tag never gets tendered, because it ought to be cost-prohibitive.I think they must rarely mention it in articles because it is designed to be punitive to the team to use it. So no one seems to even address it.
From what I can see teams have only used a third tag twice since its inception. Looks like the Seahawks tagged a left tackle named Walter Jones and St Louis Rams tagged Orlando Pace also a left tackle. Ironically the same position.
Ptofootballnetwork.com says, “No player has been tagged three times since the 2011 collective bargaining agreement (CBA) was enacted.