I think they might get it from whatever their other stance does as well. Like Valara is ranged so she gets a marksman and a fighter. Mordian is fighter and tank. Possible way to test might be to take a tank out so you only have two and see if you still get the bonus for having three tanks?

Yeah, The fact that it does nothing for her most used and important stance is a joke. They should buff her exclusive to either give more attack or add some extra penetration to is or something almost anything!