Clarence House, the official account of Charles and Camilla, shared the photo to its stories as well, writing, "Wishing Prince Louis a very happy third birthday," along with a birthday cake emoji.
To celebrate Louis' 2nd birthday last year, his grandfather Charles shared a previously unseen snap of little Louis giving him a hug.
Prince Harry and Meghan Markle are yet to share any public birthday wishes for their nephew.
Louis, fifth in line to the throne, celebrates his birthday at the same time as starting at Willcocks Nursery School in London, the same preschool attended by his big sister Charlotte.
It's the perfect choice for Louis, since his older siblings Prince George, 7, and Princess Charlotte, 5, both go to Thomas's Battersea primary school close by.
Kate has also previously shared snaps of Charlotte and George on their first days of nursery school. Charlotte started at Willcocks Nursery School in January 2018, while George went to Westacre Montessori School in Norfolk, starting in January 2016. Louis' later start date was probably the result of the latest UK lockdown amongst the ongoing Covid-19 pandemic.
It's the latest photo of several personal family snaps taken by Kate. The bike Louis is pictured riding is designed to help kids balance. It quickly sold out in England after the photo was published.
Sources told The Sun that William and Kate had chosen the Tadpole+ bike as it was a "great first bike to help your child to achieve the necessary balance to cycle," according to the company's website.
