From the Super Mario Wiki, the Mario encyclopedia
Junior Triumphant! (also referred to as Challengers Bonus 3 and Challengers +3) is the third Challengers bonus episode in the Story Mode of the Nintendo 3DS version of Mario & Sonic at the London 2012 Olympic Games. The episode is unlocked after completing Junior, Super Hero?, and unlocks Bowser's Blues and Dr. Eggman's Robots once it is completed.
Bowser Jr. and Goomba arrive at the Main Stadium and watch Metal Sonic training with Dr. Eggman. Dr. Eggman is confused by the fact that Metal Sonic is not achieving the desired speeds after being rebuilt, and sets him running again. Goomba encourages Bowser Jr. to challenge Metal Sonic after defeating a regular robot, and they head over and ask Dr. Eggman if Bowser Jr. if he can challenge Metal Sonic. Dr. Eggman tells him to leave as he would be no match for Metal Sonic, but Bowser Jr. claims that he is now as strong as Bowser. Dr. Eggman accepts the challenge, on the terms that if Bowser Jr. loses he will become Dr. Eggman's servant. Despite Goomba trying to dissuade him, Bowser Jr. accepts, on the condition that Dr. Eggman will tell Bowser how well he did if he wins.
Bowser Jr. defeats Metal Sonic, angering Dr. Eggman who agrees to tell Bowser about Bowser Jr.'s performance, although he still cannot understand how his modifications to Metal Sonic failed. He attempts to perform a data scan on Bowser Jr. to gather more information, though Bowser Jr. refuses this as it was not part of their original deal. Bowser Jr. then asks Dr. Eggman for compensation for trying to trick him, which shocks Dr. Eggman at first, however Bowser Jr. only asks for some juice and a snack, and Dr. Eggman heads off to get it for him. Goomba congratulates Bowser Jr. on his performance, and Bowser Jr. asks Goomba to also tell Bowser how well he did.