Cambridge No 3 might surprise us

Britain's Prince William and Kate, Duchess of Cambridge pose for a photo with their newborn baby son. Photo / AP

Everybody knows the phrase "an heir and a spare" - but what about the "royal third"? Set to be fifth in the line of succession, the third child of the Duke and Duchess of Cambridge is very unlikely to ever come to the throne - and when George and Charlotte have children, he'll be pushed even further down the tree.

So what does history tell us about the role of the royal third? And what might the new Prince learn from those who have been before him?

Traditionally, the third child has tended to be the charm. Those in Britain's past included Henry VIII, Edward VI, James II and William IV.

Back then, life expectancy not being what it is today, the third had a better chance of inheriting. Indeed, three was deemed a riskily small royal family. Thus Queen Charlotte, wife of George III, had 15 children. Queen Victoria had nine, and might have had more had Prince Albert lived. Charles I and Henrietta Maria also had nine. Added to this were various illegitimate offspring.

The golden era of the royal third was the Tudor period. Henry VIII was not cherished by his parents and some distance from the throne. But this stood him in good stead when - after his elder brother Arthur died - he leapfrogged his middle sister Margaret, according to the laws of primogeniture, and ascended to the throne. While Arthur had been schooled in the art of government, Henry had learned to take nothing for granted and was constantly on guard.

But it was William IV, the third son of George III, who was perhaps the most influential royal third in British history; widely credited with paving the way for the modern British monarchy.

Kate, Duchess of Cambridge with their newborn baby son as they leave the Lindo wing at St Mary's Hospital in London. Photo / AP

Charming and affable, with no need to produce an heir, he was known for his partying and vocal interest in politics.

But in 1830, at 64 - having outlived his two older brothers George IV and Prince Frederick, Duke of York - he became king; the oldest monarch to come to the throne (so far).

He provided another lesson for aspirational royal number threes: stay alive the longest and you might be rewarded.

Queen Victoria's third child, Princess Alice, took a keen interest in medical issues and alarmed her mother with her fascination with gynaecology.

She married Prince Louis of Hesse and is the Duke of Edinburgh's great-grandmother.

And so to the most recent royal third, Prince Andrew. His mother, the Queen, has always been devoted to him but, as his older brothers have neither caught tuberculosis, been killed in battle nor imprisoned in the Tower, he has only the tiniest likelihood of ever coming to the throne.

After his birth in 1960 it had seemed as if the days of the royal third might be over. All the other royals since have had just two children apiece - the Cambridges are the first to have three for 59 years. The Duchess herself is, of course, the eldest of three.

A friend once told me she wouldn't consider having a third because, "with three, there's always an odd one out."

But more often than not, number three has tended to end up on top. It's not hard to imagine the new little Prince might do something very surprising.

Illustration / Rod Emmerson