I'm part of a triad, and we all sleep together in a king bed.
Partner #1 MUST sleep against a wall. He can't sleep otherwise- we have literally moved the bed in a hotel so he can have a wall to face into.
Partner #2 sleeps in the middle- she's big on cuddles, and likes being sandwiched between us.
I sleep on the outside, always on the side that allows me to face out. Before we moved that was the left side of the bed (if you're standing at the foot), laying on my right side. Now, it's the right side of the bed, laying on my left side.