By Laura Smith-Spark, CNN

(CNN) -- Princess Charlotte -- now the middle child of Prince William and Catherine, Duchess of Cambridge -- celebrated her third birthday on Wednesday, nine days after the arrival of the newest addition to the British royal family.

Charlotte stole the show last week when she waved to the gathered media outside a London hospital as she and older brother Prince George, 4, visited their baby brother for the first time.

Kensington Palace marked her birthday with a tweet that included an image of Charlotte taken on the day she started nursery school in January.

The princess, who is fifth in line to the throne, had already won over fans on a royal tour to Germany last summer, when she cheerfully greeted dignitaries and accepted a baby bouquet of flowers.

Charlotte is widely expected to be a flower girl at the wedding later this month of her uncle, Prince Harry, and Meghan Markle, as she was for the wedding of her aunt Pippa Middleton last May. Prince George was a page boy then and is likely to repeat that role.

It's not yet known if their new baby brother, Prince Louis, will attend the wedding in Windsor.

Queen Elizabeth II visited the youngest of her six great-grandchildren at Kensington Palace on Tuesday, according to UK media reports. On the same day, his parents formally registered his birth.

The birth certificate, which lists his full name as His Royal Highness Prince Louis Arthur Charles of Cambridge, is signed by his father with the single name "William."

William and Kate ended days of speculation when they announced the name of their third child on Friday. They celebrated their seventh wedding anniversary on Sunday.