Act 3 is shorter than the previous acts and culminates in your first encounter with Dr. Robotnik. The level itself is a quick dash through familiar Green Hill scenery. Focus on collecting as many rings as possible before the boss fight, as they will serve as your health. There are usually a few ring monitors and a shield monitor available just before the boss arena.
Boss: Egg Mobile (Wrecking Ball). Dr. Robotnik will appear in his Egg Mobile, swinging a wrecking ball. To defeat him, you need to jump and hit the Egg Mobile itself (not the wrecking ball) when it's vulnerable. Robotnik will swing the wrecking ball in an arc. Wait for the wrecking ball to swing away, then jump and hit the Egg Mobile. He will usually swing it three times before pausing, giving you an opening. After taking a hit, he'll speed up or change his pattern slightly. Repeat this process eight times to defeat him. Be careful not to get hit by the wrecking ball, as it will cause you to lose all your rings. After defeating him, he'll flee, and you can free the animals from the capsule.