The Three Forgotten Brothers
From the Super Mario Wiki
The Three Forgotten Brothers is a post-game level in the Bowser Jr.'s Journey mode of Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey.
Bowser Jr. and Roy Koopa are both lying around out of boredom when the Koopa Paratroopa Trio spot them and prepare to take a shot to get in good with Fawful. However, they are reminded that Fawful has already been defeated, and Roy berates them for wearing sunglasses in front of him. The Koopa Paratroopa Trio then proceed to fight Bowser Jr. out of pride. However, the battle's aftermath is not shown, as the narration says it's better to forget the whole thing. The Koopa Paratroopa Trio do join as allies however.