Sonic 3 is a fantastic old-school gem.
You'll be able to control either Sonic, Tails, or both via co-op mode with another player as you run and jump through 8 zones; each containing their own unique theme, enemies and obstacles. Sonic 3 is a very challenging game in its own right. The bosses and mini-bosses for example, have real tricky attack patterns that you must follow carefully because they can the damage the stage (which the majority of them do including the final boss) and that will dramatically limit the battle area, making jumping and dodging even more hazardous. Even the stages can lead to deaths: moving walls are common and fast, collapsing bridges usually lead to spikes at the bottom, and confusing dead ends (mainly in the Carnival Zone) can lead to frustrating moments where the answer isn't always plain as day. To put it simply, Sonic 3 is the hardest game in the series on the Genesis (not counting Spinball).
Luckily, Sonic 3 is also fair. This is the first game in the series (and the last one until Sonic Adventure) that has a save option, which is an extremely welcome addition which lets you pass a zone and save, and then finish it another day, without finishing the game all at once like the other games in the series forced you to do. The save feature is so useful, I wonder why it wasn't included in Sonic & Knuckles.
Sonic 3 is a perfect example of ingenious gameplay and level design. Like Sonic 1 and 2, Sonic 3 features a nice balance of fun platforming and pure speed runs. Sonic 3 is, simply put, a blast to play. There's always something new and interesting in a different zone, and alternate routes in each act beg for replay value. At the time where platformers were a dime a dozen, Sonic 3 stood proudly with its excellent level design.
A cool bonus feature for Sonic 3 as was with Sonic 2 is the ability to Lock-on with Sonic & Knuckles, allowing you to play as Knuckles through the main game. You don't play as the anti-hero role as Knuckles, but more of the hero role as the spiky gloved Echidna. I could complain that it doesn't follow the storyline of the main game (Knuckles was of course on the bad side), but I was discovering a lot of cool new areas and secrets with his handy glide ability to care.
Sonic 3 is in one word: a masterpiece. It's a loyal sequel to Sonic 2, which was also an excellent platformer, so to be able to nearly best such a fine predecessor is a great feat in itself. I feel like I'm repeating sentences when I'm saying this, but it's juts a Genesis cla$$ic that I can't help myself. If you haven't played this game yet, then you're either broke or crazy. Sonic 3 is available on Virtual Console, and collections on various consoles like the DS, PS3, and past gen systems.