Warning: Contains SPOILERS for Sonic the Hedgehog 2!
It has already been confirmed that Sonic the Hedgehog 3 will feature Shadow the Hedgehog, but it is likely that some other game characters will be debuting as well. The recently released Sonic the Hedgehog 2 showcases both Tails and Knuckles, who were mostly absent from the original movie. It can be expected that Sonic 3 will be even bigger and better, though, and there is a long list of characters from the series that could appear.
Starting as a game released in 1991, the Sonic series has become a massive hit across several mediums in the three decades since its inception. On top of the over 100 games, Sonic has spawned spinoff TV shows, comic books, toys, and more. Because the series has dipped its toe into about every medium possible, Sonic has understandably added a lot of characters over the years, many of which may appear in Sonic 3.
While the post-credits scene of the first Sonic movie only features Tails, Knuckles actually ends up playing a major role in Sonic 2 as well. Similar to the first film, the sequel's post-credits teases Shadow the Hedgehog, a major antihero from the series. However, Sonic 2 shows that the movies aren't restraining themselves to only what was teased in the previous film, as Knuckles is a big enough character to have had his own post-credits scene. Because of this, it is very likely that Sonic 3 will debut other game characters besides Shadow, whether they are related to Shadow's story or are just incredibly popular.
Amy Rose
While Tails is Sonic the Hedgehog's most well-known friend, Amy Rose is a close second. This pink hedgehog wields a hammer as big as her attitude, crushing any enemy that stands in her way. One of her defining character traits is the massive crush that she has on Sonic. Sonic's main arc in both films so far has been about finding family, and giving him a love interest in Amy could build off of that. While she may not tie into the Shadow storyline and some of the other potential characters, Amy Rose is by far the most popular Sonic character that hasn't been in the movies yet, so it would be no surprise to see her appear in Sonic 3.
Metal Sonic
Metal Sonic is the second most prevalent villain in the Sonic series, behind Dr. Eggman. It has been speculated since the first film that Metal Sonic would be Sonic's villain in one of the movies, and Sonic 3 is the perfect opportunity to do this. Shadow has always been an antihero, meaning that he will probably end up teaming up with Sonic near the end of Sonic 3. Metal Sonic would be a perfect threat, as he is so powerful that it may require Shadow to team up with the other heroes. There are various ways in which he could pop up, whether he be Dr. Eggman's invention or from the GUN organization. However they do it, Metal Sonic would be an incredible addition to Sonic 3.
Rouge the Bat
Rouge the Bat is a jewel thief that often works with the Guardian Unit of Nations (GUN), a group that appeared in Sonic 2. Because of this association, she often plays a big role in Shadow's origin, a story that is teased in Sonic 2's post-credits scene. While she doesn't have as wide of an appeal as Tails or Knuckles, Rouge is still pretty popular with fans of the Sonic Adventure series, and having her pop up in Sonic 3 would be a treat.
The Chao and Cream the Rabbit
Sonic 2's plot revolves around the Master Emerald, which in the movie series is made up of the seven Chaos Emeralds. At the end of Sonic 2, the Master Emerald breaks, and the Chaos Emeralds fly off. Sonic 3 will likely require Sonic and his friends to collect the Chaos Emeralds, meaning that they might run into a species known for guarding these powerful items: the Chao. Being heavenly tied into the emeralds and the Echidna Tribe, it would be no surprise if Sonic 2 secretly set up the Chao. Cream the Rabbit often appears alongside the Chao, acting as almost a parental figure for some of them. So, if the Chao appears, Cream probably will too.
Silver the Hedgehog
Silver the Hedgehog is definitely a more outlandish pick, but Sonic 3 could easily use the time traveler in a very clever way. At the end of Sonic 2, a GUN soldier mentions that the project which created Shadow happened 50 years ago. This doesn't make sense, though, as that means Sonic wouldn't have shown up until well after Shadow was created. While it is possible that these scientists just happened to make their ultimate lifeform a hedgehog as well, it is unlikely. Time travel, a power that Silver wields, would be a perfect explanation for this seemingly odd timeline, and having him appear could fix this mess.
Maria Robotnik
Maria Robotnik's time in the Sonic series is short-lived, but it is essential to Shadow the Hedgehog's future story. Born in a massive facility in space, Maria is the cousin of Dr. Robotnik (aka Eggman). Due to an illness, she could never leave the station and go to Earth. Because of this, Maria's grandfather started Project Shadow in order to find a cure. Although it was unsuccessful, the project produced Shadow the Hedgehog, who became good friends with Maria while they lived in space. Later, when GUN attempts to shut down the project, Maria gets killed by a soldier while trying to help Shadow escape. This death had a major effect on Shadow, influencing many of the choices he makes in the present. While this seems dark for the Sonic movies that currently exist, Maria is key to faithfully adapting the Shadow storyline and thus should appear in Sonic 3.
Professor Gerald Robotnik
With Sonic the Hedgehog 2's ending setting up Project Shadow, it would only make sense for Sonic 3 to feature the experiment's creator: Professor Gerald Robotnik. Gerald Robotnik is the grandfather of Dr. Eggman and plays a major role in the story of Sonic Adventure 2. Since it is already confirmed that Shadow was created 50 years ago in the movies, that timeline matches up with Gerald being his creator. While not as evil as his mad scientist grandson, Gerald would bring the heart to Sonic 3, attempting to save the life of his granddaughter Maria in vain.
The Sonic the Hedgehog movies have only scratched the surface of what the series has to offer, as there are so many more characters from the games that would make great additions to Sonic 3. To properly execute all of the storylines from the games, the Sonic movies would have to add a lot of characters. While they may not all appear, it is very likely that Sonic the Hedgehog 3 will feature at least some of these characters from the games.