A superb game, even without Sonic & Knuckles. However, if you want the game to be perfect, you need to own both.
Sonic 3 added a few new gameplay options, such as an option to save your progress and allowing Tails to fly. However, you still couldn't play as Knuckles unless you had Sonic & Knuckles. However, this wasn't that much of a loss, since Knuckles was the bad guy in Sonic 3 besides Dr Robotnik of course.
New element shields have been added in Sonic 3. One of them is the bubble shield, which allows Sonic to bounce up and down and breath underwater. The fire shield, which protects Sonic from fire attacks and allows him to rush like a fireball. The final shield, the Lightning Shield protects Sonic from electrical attacks and allows him to pull off a double jump. It also has a magnet which attracts rings to you. All these shields are useful, and provide a much better Sonic experience.
A new special stage is in Sonic 3, which is called Blue Sphere, where you must collect all of the blue spheres and avoid all of the red ones. Once you got all of the blue spheres, you are awarded with a chaos emerald, and once you get all of these, you become Super Sonic! The ring portals are a way of reaching the special stages.
Of course, the gameplay is the most important part of the game, and Sonic 3 doesn't disappoint there! There are 6 zones with 2 acts each to blast through (14 zones if you have Sonic & Knuckles). Each act ends with a boss to fight, and each act is fairly long. Angel Island zone is the zone you start off on, where Knuckles steals your chaos emeralds. Angel Island zone is a large zone which involves a rather large jungle filled with water. Once you got past that you had to get through Hydrocity Zone, which is a city full of water (think of Labyrinth zone), then you blast onto Marble Garden Zone, which is a large garden zone full of garden traps and frisbee objects for you to use. The boss is odd on this level, since you have to use Tails to kill Robotnik. After defeating Robotnik you land into Carnival Night zone, which is a zone similar to Sonic 2's Casino Night zone without the slot machines. After using a cannon you launch into Ice Cap zone with a snowboard (Sonic only), which is a cold zone with a very catchy tune. Finally, the Launch Base zone, Dr Robotnik's launch base where he is planning to launch the Death Egg. Can you stop him in time, or will he succeed?
Now the graphics engine used in Sonic 3 is amazing! I thought Sonic The Hedgehog had great graphics but this has topped it all! All of the sprites have had a great graphical improvement and the detail in the zone environments is amazing! I just love how the background moves with your character. It's so cool and yet simply great! As for the sound, you get the usual Sonic sounds but there's also some rather cool music in this game as well. Each having their beats and memorable moments.
So I think it's become quite clear that I like this game alot. If you own Sonic & Knuckles, then you have to buy this to finish the story. If you own Sonic 3, then you need to get Sonic & Knuckles as well. Having both games in your collection unleashes the complete Sonic 3, and it's not worth missing at all. Don't get me wrong. It's a superb game, even without Sonic & Knuckles. But you really need both to appriciate the game more.
Now having both Sonic 3 & Knuckles will give you the absolute best of the best! Sure, you might think Sonic 3 is good on its own but you don't have the whole game! Make sure you have Sonic & Knuckles too to have the "Complete Sonic 3", otherwise you don't really have a full game. Don't miss out the greatest Sonic experience ever!