Intended to be the second half of Sonic 3, but you could make the case that this released as the fourth mainline game due to its standalone capability.
Sonic 3D Blast
Sonic the Fighters
Sonic Blast
Knuckles' Chaotix
Sonic the Hedgehog: Triple Trouble
Not as much as the fan remake, but Knuckles does reappear…
Sonic.EXE
joke entry obviously, but i mean… it uses sonic 3 gameplay sprites, it introduces a new playable character and brings back all the existing ones… technically!