Why can't I use Super Sonic in Sonic 3 even if I have all 7 emeralds?

You should be able to. After you collect 50 rings, double jump to turn into Super Sonic. It won't work if you have any kind of shield, you have to lose your shield first.

Also, if you are playing Sonic 3 & Knuckles, and you get to Mushroom Hill Zone (which is the first Sonic & Knuckles stage), you are able to find the Super Emeralds...and you lose your ability to become Super Sonic. To become Super Sonic again, you need to find all seven Super Emeralds. After you find the seven Super Emeralds, double-jumping with fifty rings will actually cause you to become Hyper Sonic (which is more powerful than Super Sonic).