Act 3 is a unique boss encounter, as you'll be chasing Dr. Robotnik through a vertical underwater shaft. The level itself is a continuous descent, with Robotnik dropping bombs and trying to block your path. Collect as many rings as possible at the start, as there are no rings during the chase.
Boss: Egg Mobile (Underwater Chase). This isn't a traditional boss fight where you hit Robotnik directly. Instead, you must chase him down a long, vertical underwater shaft. Robotnik will drop bombs that create explosions, and he'll also activate crushing blocks to impede your progress. Your goal is to keep up with him while avoiding the bombs, crushing blocks, and most importantly, finding air bubbles. There are strategically placed air bubbles throughout the shaft; miss too many, and Sonic will drown. Use the Spin Dash to gain speed when possible, but be careful not to overshoot air bubbles. The chase ends when you reach the bottom and Robotnik flees, allowing you to free the animals. This boss is more about endurance and navigation than direct combat.