What are the ticket rules for 3years old child?

for domestic routes, the child under 120cm can board for free if sharing the seat/bed with adult. Each adult only allow to take one child for free. If the child need own seat or bed, you can buy a child ticket then. The child between 120cm and 150cm also need to buy child ticket. Child ticket price is 50% of full price for seat and 75% for sleeper bed.