I remember playing one unofficial (i.e. not affiliated with Sega) Sonic game as a kid on Windows, but I can't find it any more. However, I've recovered a screenshot of it (I hope it is it).
Anyone knows which game this screenshot is from?
Otherwise there is another Sonic game called Sonic Dimensions (I couldn't find a link for it you'll have to search).
Hopefully you'll find it among one of those, since one of the games is a trilogy.
There's also an old "hacked" version of a Sonic game that makes you play with some characters in a different way... But I truly can't remember, this was the best I could find. You can also check out this YouTube playlist of unofficial sonic games.