Re (from Ambyer
"Just out of curiosity, what did you do, or how did you overcome, the feeling of being a third wheel?"
Well the thing is, I didn't really do anything, and the only how was the passing of time. It just started to be less of a big deal to me. It helped that more trust was built up between the three of us, so that I became more confident that I was considered to be an equal by my companions. Then it didn't seem to matter so much what the outside world thought.
I guess I had one big advantage. I always lived near my companions (just ten minutes away), or with them in the same domicile. It makes for much more opportunities to get comfortable with them in their house.
For what it's worth, when we go out to eat, we just say "table for three," and don't specify "who the couple" is.
"I realize it's mostly in my head but it just sucks knowing that say for instance, something were to happen to one of them and they in the hospital, the other would have a right to see them in ICU being the spouse, and I stand a chance of not even being allowed in since I'm not legally related in any way."
For that you need legal counsel, draw up special power of attorneys and stuff like that. That's about the extent of my knowledge; the lady of our household is the expert on legal, financial, and technical things.