Stand at the right side of the screen, not left.
Jump over the rocks when they fall down, not when they bounce up.
Honestly.
I'm not sure which Sonic 2 you were playing, because in my copy, there's no safe place where you can just stand and jump over low balls, as the high balls bounce off different positions.
As I said at Retro, you can quibble about what level of difficulty is acceptable, but it's pretty factual that there is a much lower margin of error on Game Gear Sonic 2 act 3s than any other Sonic game. I mean, the first boss of the 8-bit version puts the final boss of the 16-bit version to shame in difficulty.
And, well, look at Green Hills 3. You have no idea where you're going or where you'll land, and if you land in the wrong place you die! The only way you can call that a "cake walk" is if you've played it enough to have memorized the proper input or layout, and at that point I don't think you can accurately judge difficulty.