When I started this series I came in partway through the suggested reading order, so while this is the third I’ve read, it’s actually the eighth…. I’ll go back to the beginning once I get to the end!
It seems to overlap a fair amount timeline-wise with The Constant Princess and The King’s Curse, but takes you to mostly a completely different set of events. It’s narrated by Margaret Tudor, the older sister of Henry VIII, and the three sisters/queens referred to in the title are herself, as she marries the king of Scotland, her younger sister Mary who marries the king of France, and Katharine of Aragon, wife of Henry VIII, so sister-in-law, and queen of England. The focus is on Margaret’s story, but with written correspondence with her two sisters in London to include theirs.
Margaret is sent to Scotland as a teenager to marry their King James, over the many many years covered, she ended up having three marriages (died, divorced (and very off and on until the divorce was final), survived), and five children, only two of whom survived infancy.
Obviously there is a fictional element to the book, but the author puts a note in the end to discuss the historical elements, which was interesting (and I’ve had a go through some Wikipedia pages too to find out a bit more!). I found this paragraph from the author’s note particularly interesting to show why she told the story in the way she did:
“[…] I was struck by how their histories intertwined and reflected each other. They all three experienced arranged marriages, were widowed, and remarried the men of their choice. They all three lost children in infancy. They all three depended on the goodwill of Henry VIII, they all three fell from his favour, all three were threatened by the rise of Anne Boleyn. They were all born princesses, but experienced debt and even poverty.”
At some points it felt a bit repetitive, in and out of relationships, and so many lost pregnancies and children between the three women. Much as these are obviously really sad things, in a novel I guess it can just get a bit “oh that again” when it happens so often, but sadly a reality of that time.

Anything to add...?