"Taken earlier this week by The Duchess before he left for his first day of nursery, The Duke and Duchess are pleased to share a new image of Prince Louis," the Cambridges shared on Instagram.
In the pic, Louis posed on a red bike and wore shorts and a navy sweater over a striped shirt.
The tot is reportedly attending London's Willcocks Nursery School, which is the same school Princess Charlotte goes to. "They chose that nursery because they thought it would be an ideal first step for Charlotte's education, and they were impressed by the team who work there," an insider previously told PEOPLE.
"3?! He was only born the other day," another follower of the royals said, while a third user said: "Oh my goodness he is absolutely adorable. Just like his big brother and sister."
His birthday celebration is bittersweet for the family, having just mourned the death of Prince Philip. "Philip adored his great-grandchildren," an insider recently told OK!, "and little Louis in particular always made him chuckle."
Louis turned three on Friday, April 23, and his parents plan to treat guests to pony rides, a puppet show, a bubble-blowing contest and his great-grandmother Queen Elizabeth II will have her pastry chef prepare the birthday cake, an OK! insider revealed. "Word is, it will be decorated with marzipan zoo animals," the insider explained. “Louis loves anything to do with nature.
"The queen is always so thoughtful when it comes to presents for her great-grandchildren," the source added and "her birthday treat for Louis will be no exception.
"This celebration is a bright spot for her, and she wants to make it special for the young Prince Charming who’s making her smile again."
The picture comes just days after the Queen's 95th birthday as well as the recent death of her husband earlier this month.
"I have, on the occasion of my 95th birthday today, received many messages of good wishes, which I very much appreciate," the Queen wrote in a statement released on Wednesday, April 21. "While as a family we are in a period of great sadness, it has been a comfort to us all to see and to hear the tributes paid to my husband, from those within the United Kingdom, the Commonwealth and around the world."
"My family and I would like to thank you all for the support and kindness shown to us in recent days," the statement concluded. "We have been deeply touched, and continue to be reminded that Philip had such an extraordinary impact on countless people throughout his life."
"I will never take for granted the special memories my children will always have of their great-grandpa coming to collect them in his carriage and seeing for themselves his infectious sense of adventure as well as his mischievous sense of humour!" William wrote in a statement along with a picture of Philip with Prince George.