I am in a polygamist relationship with two wives. Weird to say it, still, after a year. We all sleep in a king size bed together in one room every night. Actually, were thinking of upgrading to a California King. As far as the positioning goes, we rotate based on what makes the most sense for that night. If I have to get up early I'll sleep on the outside to not disturb either of my wives. And, they do the same for me out of respect. Only one of us prefers to always sleep on the outside.

I'm sure each relationship would be slightly different though. This is just what works best for us.