Prince Charles laughed as a well-wisher shouted to him, "Congratulations Sir, it's triplets!"
As the Prince walked over to say hello, John Dowson joked, "I heard it on the radio".
Mr Dowson said afterwards, "He just laughed, he was really jolly. Everybody's so excited in the country for him".
More top news
Temperatures will reach around 28 Celsius (82F) at their highest.
A minute's silence will be observed at 11am on Thursday in remembrance of those who lost their lives in the Manchester Arena attack.
New research has found those who consumed two cups had a 35% reduced risk, and for those who drank five cups, the risk halved.