Was it at all possible to improve on Sonic 2? This game answers that question with a very firm "yes".
When I say the levels are HUGE, I mean... gargantuan. At the opening of the game, you have all the Chaos Emeralds taken from you, you have to pursue Knuckles, and you get caught in a massive firestorm (which looks amazing, by the way). And that's just the first act! The second act kicks things up a notch, with a section where you are pursued by a ship dropping bombs (immediately before the boss fight), and you are dropped into an aqueduct. Amazing, huh? And all of this without any slowdown!
Oh, and there's Tails. Can't leave him out. He can now fly and swim as well.
Sonic 3 (when it's locked-on to your Sonic and Knuckles cartridge) is an unbelievable accomplishment in the history of games, and is simply a must-own. Buy it now, you won't regret it.