St. James's Palace said that all three will play a leading role in "encouraging and inspiring the British public to rally behind the 900 Olympic and Paralympic athletes" competing for Britain.
They also will use their roles to encourage greater awareness of Olympic and Paralympic values, the palace said.
The royals join 27 British Olympians from previous games who were unveiled as 2012 ambassadors by the British Olympic Association earlier this year.
In a message released by the palace, William said he feels honored to have been chosen and that his entire family is looking forward to the games.
"London hosting the 2012 Olympic and Paralympic Games will inspire so many people -- particularly the young -- to be the best they can be," he said.
The young royals will be opening the latest chapter in the monarchy's longstanding ties to the games.
Queen Elizabeth II is patron of the British Olympic Association while Princess Anne -- who competed in the 1976 Montreal games as an equestrian -- is its president, a member of the International Olympic Committee and a member of the London Organizing Committee for the Olympic Games.
Anne's daughter, Zara Phillips, is a world-class equestrian who hopes to compete in the London 2012 games.
The London Olympics begin July 27 and end Aug. 12.