Image licensed to i-Images Picture Agency. 08/07/2016. , United Kingdom. The Duke and Duchess of Cambridge and Prince George at the Royal International Air Tattoo at RAF Fairford, United Kingdom. Picture by Stephen Lock / i-Images

Image licensed to i-Images Picture Agency. 08/07/2016. RAF Fairford, United Kingdom. The Duchess of Cambridge and Prince George at the Royal International Air Tattoo at RAF Fairford, United Kingdom. Picture by Stephen Lock / i-Images

This undated handout photo taken by Matt Porteous and released on July 22, 2016 by Britain's Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge shows Prince George at Sandringham in Norfolk. Prince George celebrates his third birthday today. / AFP PHOTO / DUKE AND DUCHESS OF CAMBRIDGE / Matt PORTEOUS / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / MATT PORTEOUS " - NO MARKETING NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS

Image licensed to i-Images Picture Agency. 29/09/2016. Victoria, Canada. Royal Tour of Canada -Day six. Prince William, The Duke of Cambridge, accompanied by his wife Catherine, The Duchess of Cambridge with their Children Prince George and Princess Charlotte, join parents and children from military families at a party at Government House, Victoria, Canada, on Day six of their eight day tour of Canada. Picture by i-Images / Pool

Prince George of Cambridge leaves his sister Charlotte's christening at St. Mary Magdalene Church in Sandringham, England, on July 5, 2015. Britain's baby Princess Charlotte was christened on Sunday in her second public outing since her birth nine weeks ago to proud parents Prince William and his wife Kate. The low-key ceremony took place in a church on the country estate of great grandmother Queen Elizabeth II. AFP PHOTO / POOL / CHRIS JACKSON

Prince George of Cambridge leaves his sister Charlotte's christening at St. Mary Magdalene Church in Sandringham, England, on July 5, 2015. Britain's baby Princess Charlotte was christened on Sunday in her second public outing since her birth nine weeks ago to proud parents Prince William and his wife Kate. The low-key ceremony took place in a church on the country estate of great grandmother Queen Elizabeth II. AFP PHOTO / POOL / CHRIS JACKSON

Image licensed to i-Images Picture Agency. 19/07/2017. Berlin, Germany. The Duke and Duchess of Cambridge with Prince George and Princess Charlotte arriving at Berlin airport in Germany on the third day of their Royal tour. Picture by Stephen Lock / i-Images

Image ??Licensed to i-Images Picture Agency. 17/07/2017. Warsaw, Poland. Duke and Duchess of Cambridge Tour of Poland. L to R Duchess of Cambridge's stylist Natasha Archer,nanny Maria Teresa Borrallo as Prince William, The Duke of Cambridge, joined by his wife Catherine, The Duchess of Cambridge with their two children Prince George and Princess Charlotte arrive at Warsaw airport, Poland, for a three day visit to the country. Picture by Andrew Parsons / i-Images

Image ??Licensed to i-Images Picture Agency. 25/12/2016. London, United Kingdom. Duke and Duchess of Cambridge Christmas Day Service. Englefield. Prince William, The Duke of Cambridge, Catherine, Duchess of Cambridge, with their children, Prince George and Princess Charlotte attend the morning Christmas Day service at St Mark's Church in Englefield, Berkshire. Picture by i-Images / Pool

Image ?Licensed to i-Images Picture Agency. 20/05/2017. London, United Kingdom. Pippa Middleton wedding to James Matthews. HRH The Duchess of Cambridge with her children HRH Prince George of Cambridge and HRH Princess Charlotte of Cambridge at the wedding of her sister Pippa Middleton and James Matthews at St Mark's Church, Englefield, Berkshire, UK. Picture by i-Images / Pool

Britain's Prince William and Kate, the Duchess of Cambridge with their children Prince George and Princess Charlotte arrive to attend the morning Christmas Day service at St Mark's Church in Englefield, England, Sunday Dec. 25, 2016. A heavy cold is keeping Queen Elizabeth II from attending the traditional Christmas morning church service near her Sandringham estate in rural Norfolk, England. (Andrew Matthews/PA via AP)

Image licensed to i-Images Picture Agency. 01/10/2016. Victoria, Canada. Prince George squashes his nose against the window of the seaplane as it takes off from Victoria Harbour, Canada at the end of the Royal Tour. Picture by Stephen Lock / i-Images

VICTORIA- BC- CANADA- 1st OCT-2016 Prince William, the Duke of Cambridge and Princess Catherine, the Duchess of Cambridge accompanied by Prince George and Princess Charlotte depart from Victoria Harbour by Sea Plane to Vancouver and from there back to the UK on their last day of their Official Tour of Canada. ?Exclusivepix Media

Image licensed to i-Images Picture Agency. 29/09/2016. Victoria, Canada. Royal Tour of Canada -Day six. Prince William, The Duke of Cambridge, accompanied by his wife Catherine, The Duchess of Cambridge with their Children Prince George and Princess Charlotte, join parents and children from military families at a party at Government House, Victoria, Canada, on Day six of their eight day tour of Canada. Picture by i-Images / Pool

Prince George reacts to bubbles being blown by his father the Duke of Cambridge during a children's party at Government House in Victoria, B.C. Thursday, Sept. 29, 2016. (Jonathan Hayward/The Canadian Press via AP)

The Duke Cambridge looks on as his son Prince George blows bubbles during a children's party at Government House in Victoria, B.C. Thursday, Sept. 29, 2016. (Jonathan Hayward/The Canadian Press via AP)

The Duke and Duchess of Cambridge and their children Prince George and Princess Charlotte arrive in Victoria, B.C., on Saturday, September 24, 2016. (Darryl Dyck/The Canadian Press via AP)

The Duke and Duchess of Cambridge and their children Prince George and Princess Charlotte arrive in Victoria, B.C., on Saturday, September 24, 2016. (Jonathan Hayward/The Canadian Press via AP)

(L-R) Britain's Catherine, Duchess of Cambridge holding her daughter Princess Charlotte, Prince George, Britain's Prince William, Duke of Cambridge and Britain's Queen Elizabeth II stand on the balcony of Buckingham Palace to watch a fly-past of aircrafts by the Royal Air Force, in London on June 11, 2016. Trooping The Colour and the fly-past are part of a weekend of events to celebrate the Queen's 90th birthday. / AFP PHOTO / JUSTIN TALLIS

PAP06161440 11 June 2016. Trooping the Colour, as The Queen marks her official birthday by carrying out an inspection of her troops from the household division March in Whitehall, before watching a fly-past from the balcony at Buckingham Palace. Here, Catherine, Duchess of Cambridge, Princess Charlotte, Prince George, Prince William, Duke of Cambridge PAP061614338