Take it with a grain of salt........
I am relatively new here, but i thought i would add my own experience to the mix. Feel free to smack me in the neck if i get out of line.
My wife and I have been together for eight years. During that time our relationship, our marriage, and ourselves have changed. A lot.
One thing we have found that works is not to think of our relationship with each other as a "thing". Its not quantifiable, its not measureable, and it is not static. Our relationship is the sum of the interactions, emotions, experiences, and shared moments we have together. As we change, and we do, so does it.
I understand the distaste that comes from phrases like "adding a third" or "bringing someone in" and the like, but mostly i feel that it stems from communication issues, or a fundamental misunderstanding of a specific desire.
I dont see it as possible to "bring in" another to an existing relationship, simply because the presence of another person in that relationship will change it, for better or for worse. I do believe it possible to have another individual enter the lives of a couple, and to form a new relationship dynamic, involving all three persons. But i see it as three relationships simultaneously, both seperate from, and included in, each other.
The initial relationship is still intact at this point, with two new connections to it, interconnected, yet independent. Over time, the level that those three interactions are connected will inevitably adjust, and there is no real way of knowing what shape it will take down the line. And therin lies the rub.
How can you expect to invite someone into an existing relationship when they are independent of it to begin with?
I dont see it as possible. I do think that successful triads can and do exist, but not as a single relationship. That would be like saying a successful marriage turns two people into one. They may share goals, tastes, emotions, anything really, but at the end of the day, it is still two people with two different lives who choose to live them together.
A marriage, or any other relationship is not a static thing. It grows, evolves, changes with the people in it.
So i guess, scemantics aside, i dont see a way to "add another" to a relationship. But i do see a way to add relationships to an existing couples' lives.
Good lord, i hope i am right about at least some of this, or there are going to be some serious hurts in the future, i am sure.
Life is all about connections. Love is infinite. Spread the wealth.