Third Wheeling It

You've known them since you were six months old.
You've spent your entire childhood with them: summers and winters and trips rolled into one mega coaster that can only belong to you. The three of you are closer than siblings, and you know each and every single thing about one another. (You might have liked one of them.)
It's been twenty-five years, but you were always together: a group of three. A trio.
Until it's a sunny evening in June and you're watching your bestie and former crush get married...