Wait, that's an interesting one. I'm pretty sure you're referring to a Wimpy Kid book, right? The seventh book in the main series by Jeff Kinney. The 'third wheel' part really is the key. So the main characters are, obviously, Greg Heffley as the narrator and protagonist, and his best friend, Rowley Jefferson. The whole plot kicks off because Greg needs a 'date' for the school dance and ropes Rowley into it, making them a weird duo.
Most of the conflict revolves around their friendship getting strained when a new kid, a girl named Abigail Brown, gets involved. She becomes the 'third wheel' that the title mentions, but honestly, she's more of a catalyst. Greg's family—Mom, Dad, Rodrick, and Manny—are all in there too, driving him nuts as usual. The core dynamic is always Greg and Rowley, with everyone else orbiting their chaotic middle-school universe.
It's funny how the simplest premises in these books create the biggest messes for Greg. The 'strikes back' part is pretty literal, with his plans backfiring spectacularly, as always.