Sonic 3 is pretty faithful to Sonic Adventure 2, but it does make one big change.
Sonic the Hedgehog 3 borrows heavily from Sonic Adventure 2, a video game originally released for Sega's Dreamcast console. The movie features many of the same story beats that appear in that game, but it does take some liberties with the source material. One of the most notable changes is the role of Gerald Robotnik. In the game, Gerald only appears in flashbacks and video footage, as the character had long been dead. However, Gerald is alive in the new movie, and we get to see him interacting with his grandson, Ivo, with both characters being played by Jim Carrey. In an interview with ComicBook, director Jeff Fowler explained the motivations behind the change.
"I feel like it would be a crime against the world if I had Jim Carrey playing two characters and they never shared a scene together. It just felt like a horrible waste of comedic talent. And really that was my pitch to Jim about the whole endeavor: 'what if we explored family from Robotnik's side of things?' We'd done it in the first two films with Sonic, it was such a strong part of that messaging, but we had never asked the question with the villain, with the Robotnik side of things," said Fowler.
"What would it mean to this character of Robotnik, who has turned his back on humanity, and has shunned it, and has retreated into the arms of technology? What would it mean if suddenly a grandfather walked into his life? That just opened up the doors to some really exciting story ideas and directions, and it just felt like a great new territory for us to explore."
As Fowler alludes, found family has been a major theme throughout the Sonic movies, as well as the Knuckles streaming series. When Sonic begins his journey in the first film, he finds himself all alone on Earth. However, by the end of the second movie, the blue blur has "parents" in Tom and Maddie Wachowski, and "brothers" in Tails and Knuckles. Knuckles even gets a similar arc of his own as he develops a bond with the Whipple family.
While the impact of friends and family can be seen throughout the Sonic Cinematic Universe, we never really get a sense of Robotnik having anyone to call his own. Agent Stone looks up to him, but the relationship is clearly one-sided, with Robotnik acting pretty abusive towards his henchman. The sudden appearance of Gerald in Sonic the Hedgehog 3 gives us a lot more insight into who Ivo is at his core. From that perspective, it makes a lot of sense why Fowler and the filmmakers decided to make Gerald such a key part of the movie's narrative!