Thread:BlueSpeeder/@comment-3113156-20150709003623/@comment-4533965-20150709015216

There isn't official material stating when it takes place, though by using hints with the story and context clues, Sonic CD is most likely to take place after Sonic 1. The reason I would say that is because at the end of Sonic 2, we see Sonic and Tails flying above the ocean, and we can assume they kept flying until they got to Angel Island. Further proof goes to Sonic turning super as they get to Angel Island at the beginning of Sonic 3 and the Death Egg crashing nearby, probably landing on Angel Island.

However, that's all just speculation, so I'm probably wrong. There isn't an official source or a timeline. Sorry if I couldn't help you.