Sonic the Hedgehog 3 is a 2D side-scrolling platformer. At the start, players can select Sonic, Tails, or both. In the latter choice, players control Sonic while Tails runs along beside him; a second player can join in at any time and control Tails separately. Sonic 3 adds the ability for Tails to fly for a short time by spinning his twin tails like a propeller; when he gets too tired, he falls. Unlike Sonic, Tails can also swim underwater.
The game takes place over six zones, each divided into two acts. Levels are populated with Robotnik's robots, called "badniks"; Sonic and Tails can defeat badniks by jumping on them or using the "spin dash" attack, which also gives the character a speed boost. The levels include obstacles and other features such as vertical loops, corkscrews, breakable walls, spikes, water that the player can drown in, and bottomless pits. There is a miniboss fight with one of Robotnik's large, powerful robots at the end of the first act of each level and a full boss fight with Robotnik at the end of the second. Reaching a new level saves the game to one of six save slots.
As with previous Sonic games, Sonic 3 uses rings, scattered throughout the levels, as a health system; when the player is attacked without rings, is crushed, falls off-screen, drowns, or exceeds the act's ten-minute limit, they lose a life and return to the most recently passed checkpoint. Dying with zero lives gives the player a game over. The levels also include power-ups in television monitors that, when hit, grant the character an extra life, temporary invincibility to most hazards, a number of rings, a shield that allows them to breathe underwater, a shield that allows them to withstand fire from enemy projectiles, or a shield that attracts nearby rings.
The game contains two types of "special stages". When the player collects at least 50 rings and passes a checkpoint, they can warp to the first type, which involves bouncing up a gumball machine-like corridor to earn power-ups by hitting a switch. Both sides of the corridor are lined with flippers, which disappear when the character bounces on them, and the switch drops when both flippers supporting it are removed. The corridor's floor contains a bounce pad, which also disappears after one use; falling afterwards causes the player to leave the stage with the most recent power-up collected.
The second type, triggered by entering giant rings found in secret passages, involves running around a 3D map and passing through all of a number of blue spheres arranged in patterns. Passing through a blue sphere turns it red, and touching a red sphere causes the player to leave the stage, unless the player has just completed a cycle around an arrangement of blue spheres, in which case all of these spheres turn to harmless rings. Removing all of the blue spheres gives the player a Chaos Emerald; if Sonic (not Tails) collects all seven, he can become Super Sonic at will, which makes him invincible to most obstacles. Failing to collect the seven Chaos Emeralds triggers a post-credits scene in which Robotnik and Knuckles taunt the player.
Sonic 3 includes a competitive mode: two players, controlling Sonic, Tails, or Knuckles the Echidna, race through one or all of five stages that do not appear in the main game. In these same stages, a single player can compete against the clock in time attacks.
Sonic the Hedgehog 3 & Knuckles
Sonic the Hedgehog 3 and Sonic & Knuckles were intended to be a single game, but were released separately due to time and financial constraints. The Sonic & Knuckles cartridge features a "lock-on" adapter that allows other Genesis cartridges to be physically attached to it. Connecting the Sonic 3 cartridge creates a combined game, Sonic 3 & Knuckles. The lock-on function is available in some digital releases of the games, such as the Virtual Console service for the Wii .
Sonic the Hedgehog 3 & Knuckles allows the player to play Sonic 3 levels as Knuckles or Sonic & Knuckles levels as Tails or both Sonic and Tails. Other new features are the ability to collect Super Emeralds, unlocking new "Hyper" forms for Sonic and Knuckles and a "Super" form for Tails, improved save options, access to new areas that Sonic or Tails could not previously access, altered boss forms, and an additional ending that shows Sonic returning the Master Emerald to Angel Island.
After Sonic and Tails defeat Dr. Robotnik in Sonic the Hedgehog 2, Robotnik's space station, the Death Egg, crash-lands on the floating Angel Island. There, Robotnik meets Knuckles the Echidna, the last member of an ancient echidna civilization that once inhabited the island. Knuckles is the guardian of the Master Emerald, which grants the island its levitation powers. Robotnik dupes Knuckles into believing Sonic is trying to steal the Master Emerald, turning the two against each other while he repairs the Death Egg.
Sonic and Tails approach Angel Island in their biplane, the Tornado. Sonic uses the Chaos Emeralds to transform into Super Sonic, but Knuckles ambushes him and steals the emeralds. Sonic and Tails travel the island hindered by Knuckles and Robotnik. At the Launch Base, where the Death Egg is under repair, Sonic and Tails fight Knuckles, but the Death Egg relaunches. On a platform attached to the Death Egg, they defeat Robotnik, causing the Death Egg to crash-land on Angel Island again. The story resumes in Sonic & Knuckles.