I noticed some similarities between those three games. 1. All 3 had their respective soundtracks changed: Sonic CD has different soundtracks in Japanese/European version and North American version; Sonic 3's music had been changed several times: Michael Jackson team's songs replaced some themes from original version, then those themes returned in Sonic & Knuckles Collection and Sonic Origins; Sonic 3D also has two versions of soundtrack: Mega Drive version and Saturn version. 2. Second levels from CD, 3, 3D are either ruin-themed, take place underground or have water. Note: in CD's case, I'm talking about unused Dubious Depths level – it could have been a dungeon and ruins are seen in extended animation from that game. 3. Alliterative names in CD and 3D: Quartz Quadrant, Diamond Dust, Stardust Speedway, Volcano Valley etc. 4. Giant Eggman statues in CD and 3D. 5. Sonic can't become Super Sonic in CD and 3D. 6. Similar boss fight in CD and 3D.