“I think he’ll be a delight to watch for years to come.
“He’s so cute, but I would have guessed that he is six years old if I didn’t know better.
“He looks so mature in that sweater.”
Many of the Royal Family’s social media accounts also celebrated Louis’ birthday.
The official Royal Family page reposted the photo.
They wrote: “Wishing Prince Louis a very happy third birthday today.”
Prince Charles and Camilla, Duchess of Cornwall, also sent well wishes to their grandson.
Clarence House shared the photo with the caption: “Wishing Prince Louis a very happy third birthday.”