Young British royals say 'We love you, Papa' in Father's Day message

Trooping the Colour parade to honour Britain's King Charles on his official birthday, in London

LONDON (Reuters) - The three young children of British heir-to-the-throne Prince William and his wife Kate released a Father's Day message and photograph on Sunday, saying "We love you, Papa".

The photo shows the three children, Prince George, 10, Princess Charlotte, 9, and Prince Louis, 6, hugging William on a beach, with the photograph taken from behind as they all look out to sea.

The caption says the photo was taken by Kate and reads: "We love you, Papa. Happy Father's Day," followed by two red hearts and G, C & L.

The photograph was released a day after Kate, Britain's Princess of Wales, was seen in public for the first time since she revealed she was undergoing treatment for cancer.

Kate, William and the children joined King Charles and other members of the royal family on the balcony of Buckingham Palace after watching a military parade to celebrate the monarch's official birthday.

The princess, 42, spent two weeks in hospital in January after she underwent major abdominal surgery. Two months later she announced in a video message that tests had revealed the presence of cancer and she was receiving preventative chemotherapy.

She is still undergoing treatment, but she said in a statement on Friday that she was able to attend the "Trooping the Colour" event because she was making good progress, although she noted that she was "not out of the woods" yet.

In a separate post on Sunday, the couple shared a photograph of William as a child playing football with his father, Charles.

(Reporting by Kate Holton; Editing by Frances Kerry)