Spoilers ahead for Episode 8 of The Crown Season 3. There are several love triangles in The Crown's third season — one of which is especially tangled. Prince Charles (Josh O'Connor) finally meets his future wife, Camilla Shand (Emerald Fennell), the on-again, off-again girlfriend of Andrew Parker Bowles, while Princess Anne (Erin Doherty) — Charles' sister — begins seeing Andrew. The latter relationship is much more casual, and perhaps not as well-known as Charles and Camilla's.
In the show, Andrew remarks to Anne that she's "developed," and she flirtatiously responds that she's noticed him on the polo field. Later, the two have sex, but Andrew insists it should be the one and only time because he doesn't want to hurt Anne. When she asks what he means, Andrew responds, "Oh, it's what tends to happen to anyone who gets in the way," referring to his relationship with Camilla.
Biographer Penny Junor (who has written several books on the royal family) said during a talk at the 2017 Henley Literary Festival that Camilla may not have pursued Prince Charles if Andrew hadn't initiated things with Anne first, per the Daily Mail. "[Camilla] was passionately in love with [Andrew] but he was a cad, he was bonking other people, some of her friends," Junor said. "So when she was introduced to Charles and he thought she was pretty special...she thought, 'Andrew is at the moment off with Princess Anne, you know her brother, teach Andrew a lesson.' So she had a fling with Charles."
Biographer Tina Brown wrote in her 2007 biography, The Diana Chronicles, that not only was Andrew a "royal groupie," but Princess Anne "always enjoyed a roll in the hay," per the Express. "There was a romantic re-enactment of 'La Ronde' on the dance floor at Annabel's one night in 1971 when it was clear that Princess Anne was in love with Andrew Parker Bowles, Camilla was in love with him too, and Charles was in love with Camilla."
However, according to a Channel 4 documentary, Anne and Andrew couldn't get married because he was Catholic, and regardless, the couple didn't last (though they remain friends). Camilla and Andrew married in 1973 and divorced in 1995, while Princess Anne married Captain Mark Phillips in 1973, divorced him in April 1992, and married Timothy Laurence that December. Charles was married to Princess Diana from 1981 until 1996; he re-married to Camilla in 2005. "There were three of us in this marriage, so it was a bit crowded," Diana told Martin Bashir in a '95 TV interview.
In the grand scheme of royal history, Princess Anne and Andrew Parker Bowles may have just been a blip, but without their fling, Camilla and Prince Charles may not be happily married today.