The 3 groups of fish are:
Jawless, cartilaginous and bony fishes.
Jawless fish were one of the first vertebrates in this world. These fish have no scales, and instead are made of cartilage. Also, like the name says, they have no jaws and no fins. Some jawless fish include hagfishes and lamprey's. These are the only species alive today.
Cartilaginous fish include sharks and rays. Their skeletons are made out of cartilage, similar to the jawless fish. But, they do have jaws and fins. Their bodies are covered with pointy teeth-like scales, which makes their skin rougher than sandpaper.
Bony fish have scales that overlap each other, and instead of cartilage, they have bones like other animals. These scales are made from the same substance as your fingernail. The slimy substance mucus covers the fish (it's in your throat and nose as well), and protects it from other dangerous things. Bony fish are basically 95% of all fish.
