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 Generations
(the 2D half of the game)
Sonic Adventure
Sonic Robo Blast!
Sonic the Hedgehog Pocket Adventure
Sonic Advance
Sonic the Hedgehog: Triple Trouble
Not as much as the fan remake, but Knuckles does reappear…
Sonic R
Sonic the Fighters
Knuckles' Chaotix
Sonic X-treme
Sonic 3D Blast
Sonic Robo Blast
Sonic Crackers
Sonic the Hedgehog 4
No.
Sonic Jam
Sonic Blast
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!