There should be no "3rd pair". Sanderson would get PP and PK time + 20mins/night. He'd also get to take advantage of easier matchups, which would most likely beneficial for his offensive development in the NHL.
Tampa won their cups with Hedman, Mcdonagh, Sergachev on the left side. Each one of them played 20mins+/night and one was always on the ice at any point in the game. No "weak pairing" to speak of.
Chicago did the same with Keith, Hjalmarsson/prime Oduya and Leddy on the left side.
It's our best course. Don't force players that excel on the left to the right for the sake of having a "strong top 4". Roll with one of them on each pair, have one of them on the ice at any point in games and have a strong defensive core top to bottom. It's a luxury we haven't had in Ottawa... ever.
As an early Dorion believer, then hater the moment Zibanejad + 2nd for Brassard was done, it's his best work along with Stützle's contract. This trade makes up for a lot. A lot.