The mobile versions of Sonic 1 / 2 / CD were built for variable display width.
Meaning that it will compensate for some phone screens being wider than others in landscape orientation, this made it a lot easier to port the decompilations to the 3DS.
You can tinker with the width on the PC desktop builds too, but the height is fixed to 240, with appropriate scaling options.
The point anyway is that Sonic 3 AIR, I don't think was built for variable display width like this as it seems to hardcode everything for 16:9 display support.
Which unfortunately would make it harder to port the game to any devices that don't use 16:9 screens. I'd be really interested in seeing Sonic 3 AIR ported to Anbernic devices like the RG351V (4:3) and RG351P (3:2) for example.