Sonic 3's story follows on from Sonic 2. Dr Robotnik's Death Egg crash lands on a floating island known as Angel Island. There he meets Knuckles the Echidna, who guards the Master Emerald. Dr Robotnik convinces Knuckles that Sonic is out to steal his Emerald. As Sonic and Tails arrive by plane, Knuckles ambushes them and steals Sonic's Chaos Emeralds. It's up to Sonic to recover his emeralds and stop Dr Robotnik once more.
You can control either Sonic or Tails, (or two players can play simultaneously via co-op). The levels are fairly large, but there's less of them compared to previous games. There are six Zones, each containing two Acts each. When you reach the end of an act, you continue playing as if the world is all interconnected which is a really cool effect.
There's plenty of boss battles throughout the game, but boss battles aren't usually all that fun in platforming games. The majority of them in this game can be defeated with the same strategy and they just seem to exist to add a bit of variety to the gameplay.
There's frequent check-points scattered throughout the level which allow you to respawn at that position when you die. If you lose all your lives, you have to start from Act 1 of that particular Zone (rather than restarting the entire game like previous games). Modern versions of the game (such as Sonic Mega Collection) allow you to save mid-game anyway.
If you have 50 rings and pass a check-point, you can enter a bonus stage. There's also hidden large rings which take you to the special stage. This stage is rendered in 3D where you have to pass through blue orbs while avoiding the red ones.
There's a few new power-ups in the form of shields. The Lightning shield enables Sonic to perform a double jump, the Water shield lets Sonic breathe underwater and perform a bounce-attack, whereas the Fire shield allows him to do a horizontal mid-air dash.
In all honesty, I've never really seen the appeal of Sonic. People say he runs fast, but the game's design (in most games) stops you from doing so by placing enemies, spikes or springs in your path. Usually you move forward a bit and have to stop. Even if you are travelling fast, you end up randomly jumping and hoping that you actually make some progress. Furthermore, Sonic's run speed in this game seems slower than previous installments. The fastest moments in this game is where you are doing nothing: bumping into a spring which whizzes you around loops, jumps, slopes until you come to a standstill and regain control once more. Apart from being aesthetically pleasing, the level design doesn't really allow for speed or skill and just ends up being the mindless 'stumble around until you reach your destination' style gameplay. This is particularly evident in the final stage which seems like one huge maze.
If you like the early Sonic games, then you will find this to be a good game. For me, it is a step back from Sonic CD so is a disappointment.