I find it a peculiar thing to say "the" third person. Because, when there are three people, every one of them is a "third person" from a certain perspective.
I very much agree with this perspective. For example, G & K are good at strategy games. I am not. It's frustrating to play them, and I feel stupid. So, this is something they share that I am not really a part of... same with jigsaw puzzles. I hate them with a passion. He had her read The Fountainhead by Ayn Rand. I read Anthem years ago, and it was tolerable... but Ayn Rand is a pedantic writer and I just can't handle her dense books. I decided it was okay for them to have this without me... but there were times when I felt a bit left out by it.
She and I share some TV shows without him, and he and I have some without her. He and I are both atheists, while she's sort of vaguely spiritual. He and I agree about politics - we're almost exactly of the same mind on every political issue.
One big thing they shared related to sex. It's too private for sharing here, but suffice it to say it's something I can't relate to or even really understand... traumatic experiences from their past, and they shared a pretty intense moment over it - one that took me by surprise, and was really hard for me because it was something I was never able to do for him... and it's something I rationally know I don't want to share. All the same, it was a situation where I was definitely in that "third" position.
So, while I agree that the third in an established relationship is often the one with the biggest emotional risk involved, I don't think it's entirely fair to think that you are ALWAYS the third simply because you came last. It just doesn't work that way in our triad, and it shouldn't. My guess is, if you always feel like the the third, you're not in the right relationship for you. Most of the time, we feel like three... but there are times when we break down into two, and someone else is the third besides our girlfriend.