Act 3 is a short dash to the boss arena. The level itself is straightforward, with a few springs and bumpers. Focus on collecting as many rings as possible before the boss fight. There are usually a few ring monitors and a shield monitor available just before the boss.
Boss: Egg Mobile (Spike Ball). Dr. Robotnik will appear in his Egg Mobile, which is equipped with a large, spiked ball that he drops onto the ground. The arena has several platforms that rise and fall. Robotnik will fly across the top, drop the spike ball, and then retrieve it. To hit him, you need to jump and hit the Egg Mobile itself when he is low enough, usually after he has dropped or is retrieving the spike ball. The spike ball itself is dangerous and will damage Sonic if he touches it. Hit him eight times. After each hit, he will speed up or change his pattern slightly, sometimes dropping the spike ball more frequently or moving faster. The key is to use the rising platforms to get enough height and time your jumps carefully to avoid the spike ball while hitting Robotnik.