To this day, it's the very best Sonic game in the series! I'd give this a 10 if combined with Sonic and Knuckles.
The most noticeable change is the graphics overhaul: Characters have shiny new sprites and improved animation from Sonic 1 and 2 while the 6 zones are colourful, energetic and very well designed. The new special stage features a 3D world, and pushes the Genesis' capabilities. There's no noticeable slowdown, even during busy screens so the aim of speeding through levels isn't harmed. The graphics, along with Sonic and Knuckles which boasts the same sprites and similar level design, are some of the best on the Genesis.
The soundtrack is a masterpiece. Over ten years on, the music is among my all-time favourites. It's a clear step up from the music of previous games and almost every piece is memorable.
The gameplay is simple, just how other Sonic games were, and in this case, it works just as well! You have 10 minutes to speed through each act. There are 12 acts in total, each with a boss fight at the end. Sonic must also destroy enemies along the way using his jump or spin-dash move. There's a choice of 3 character combinations. Sonic on his own, Tails, who has the ability to fly and swim for a limited amount of time, or Sonic with Tails following him. A second player can control Tails in the situation and the pair can co-operate, by having Tails carry Sonic while flying. No character has an easier route through the game but Tails arguably has a better advantage.
The game feels somewhat lacking in length. There are only 6 zones and while they are well designed it pales in comparison to the 11 zones in Sonic 2. For the first time in the series' history, players can save their progress, so they don't need one huge sitting to finish the game as the case used to be. Even when players beat all the stages, there's still the emerald challenge, giving you access to Super Sonic once you obtain all 7. Finishing the game doesn't take long, while some sections are challenging it is over a bit too soon. Ignore this criticism if you use Sonic 3 with the "lockon" facility in Sonic and Knuckles, the sequel to this story. Doing this gives you a total of 14 levels to complete and offers a full conclusion to the story. It generally feels more complete than Sonic 3 on its own.
Overall, even with complaints about the length, this is an extremely memorable game I grew up playing and it's near faultless in my opinion. Even a decade later, Sonic 3 doesn't feel dated. It's surely proof that some franchises are best sticking to their roots.