Share Tweet Pin Email By Katherine Tinsley 9:38pm PDT, Aug 12, 2025 _ Princess Diana was clearly aware that King Charles III was cheating on her. She famously called Queen Camilla the third person in her marriage, and according to a new book, she once ambushed her husband's mistress at a birthday party.Keep reading for the details...MORE: Follow Wonderwall on MSN for more top news _ Tom Bower writes about Princess Diana discovering the affair between King Charles III and Queen Camilla in his new book, Rebel King: The Making of a Monarch.