More than two people in a marriage?
Why?
If a person can not get along with their current marriage partner, then how would adding a third person improve the situation?
("I am having such a rough time in my marriage. I know what will fix it! We'll take on another person! That will make things better! I just know it!")
And if both partners are already getting along, the best that they possibly can, how will bringing another person into the mix improve things?
("We get along so perfectly, so let's bring someone else into our perfection and make it even more perfect!")
Either way, it's like two people having a baby in an effort to make things better - it will either result in a bad situation becoming worse, or a good relationship between two people becoming all muddled up in the needs of the third.
Although I suppose it could work if there were a heirarchy of some kind in place, with the founding couple at the top, and every subsequent marriage partner taking on a progressively subservient role.
No thank you.