Their is a Sonic Classic DS game, which has Sonic 1, 2, 3 and Sonic & Knuckles all on one game. Plan to get it eventually.

I don't own or would ever touch a Sony console. Sega should release PS 1-4 (didn't know of a #4 until now) for the DS. They released them for the GBA, but they're not playable on a DSi.