Who were three sons of Cronus?

Zeus, Hades and Poseidon are the sons of Cronus and Rhea.

Chiron, Aphros and Dolops are the sons of Cronus and Philyra.

Aphros and Bythos are Ichthyocentauri (Fish Centaurs).

Chiron is a Centaur.

Korybantes and Picus (which may be the Ichthyocentauri) are also called the sons of Cronus.