This Instagram account is the work of Peter Alden, a 29-year-old who works at a 3D interactive software start-up in New York City.
He tells BuzzFeed, "I am 29 years old, single, and living in NYC. Help!"
The other two people in the picture are his brother, Ben, and Ben's fiancé, Marissa Evans – who is also Peter's best friend.
Peter said, "I used to live with my brother. When they started dating I was always around."
"After almost two years, they moved in together and I had to move out on my own. I tried to live with them, but nooooo."
He added: "I spend a ton of time with them and it started as me just photobombing them every time they tried to take a cute picture together."
"Everyone (especially my mother) thought it was fun and it kind of took off from there!"
He says his friends find it pretty funny. The account "ends up being the talk of a lot of parties. People want to know who takes the photo, they want to be 'a guest couple', they want to suggest 'ideas for the next third wheel photo'..."
Anyway, the couple is now engaged, and Peter is hoping to be best man, so there should be some excellent engagement photos to look forward to.
He says he's not sure how it'll end. "Maybe I find myself a girlfriend...maybe they move away...maybe I'm a third wheel for life.
Let's hope not. But on the plus side, he would have a funny Instagram account to show for it...