The photographs were taken recently by photographer Matt Porteous at the family home, Anmer Hall, in Norfolk.
The Duke and Duchess of Cambridge thanked well-wishers for the greetings they had received to mark George's special day.
A Kensington Palace spokesman said: "The Duke and Duchess hope that people will enjoy seeing these new photographs.
"They would like to thank everyone for all the lovely messages they have received as Prince George celebrates his third birthday."
The pictures, taken in mid-July, show George at play in the garden.
Photographer Matt Porteous said it was an honour to be asked to take the photos.
He said: "I really enjoyed the opportunity to take these photographs of Prince George. It was a very relaxed and enjoyable atmosphere.
"I'm honoured that they have decided to share these images with the public to mark his third birthday."
The future king, who is third in line to the throne, is expected to be the star turn at a party hosted by his parents.
Proud mum Kate describes her son as "a really lovely little boy" and a great brother to his sister Charlotte.
Last year, George enjoyed a party for his second birthday at the family home near Sandringham, the Queen's Norfolk home.
Family and friends who attended included his parents, Charlotte, grandmother Carole and great-grandfather Prince Philip.
It is hoped that Kate may have the help of her newly engaged sister Pippa Middleton at this year's celebrations.
Pippa is heavily involved in the Middleton party planning business and has even published her own book, a one-stop guide to entertaining.