William, Kate, George and Charlotte have welcomed the newest member of their family.
As Prince William took his new son home, he said there would be "three times more to worry about".
But how challenging is a third child and how does it change a family dynamic?
Linda Blair, a clinical psychologist and columnist, gives ITV News an insight into the impact of the third child:
What are the perks of being a third child?
The third child in the family is pretty lucky.
The boundaries of what they can get away with are wider because parents learn with each child how many of their rules were not necessary, which means the child has a lot more freedom.
What about disadvantages?
The third child may be slightly impatient, may be quick to give up on tasks and the rule doesn't apply to everybody, but they may have lower self-esteem.
How does a third child affect the rest of the family?
Once you have more than two children, there's an interesting shift, they tend to bond with each other more profoundly than if there is just two.
Parents matter, always, but there's good bonding within the kids, which only grows, with more children.
The most difficult transition is the first child to anybody else, because the first child is the only child in a family who has exclusive parental attention and then has to learn to share it.
And if you have more than three?
They'll be more like a gang, they'll get along well, they like each other, they'll turn to each other for support, especially number one and number four.
Sometimes if there's different genders and the spacing between the kids also influences what happens.
Is there an ideal number of children to have?
I (asked this) in clinic when people are thinking about the size of their family.
I say the happiest families are the ones where the parents are delighted with the number of kids they have.