This is because you are ignoring loser points. Sure, teams almost never lose in regulation when up 2 in the 3rd, but they sure can lose in OT, and they do. The Islanders had a 2 goal lead in the 3rd period and lost in OT just yesterday.
It is not freaking impossible to win a game down 3 goals in the 3rd. It is almost impossible to do it in regulation, which is why this isn't showing up for you.
In 1998, there were 56 wins by a team trailing going in to the 3rd period, out of a total of 1066 games total. There were also 98 ties. This season, there've been 115 wins by a team trailing in the 3rd period, out of a total of ~864 games. The scoring landscape has changed and also OT/shootouts eliminate ties, but comebacks happen far more frequently now even still.