Their Viscountess by Jess Michaels

From the time they were teenagers, Emilia, Wren and Aiden knew they desired and loved each other, but their disparate classes and her father’s intention to wed her to a title separated them cruelly for years. They had all accepted that they would likely never be together again until a shocking truth pulls them into each other’s orbit once more.
Emilia’s marriage to a viscount was anything but happy, and it has turned dangerous. She believes he’s trying to kill her and in desperation she reaches out to Aiden to steal her away and save her. Meanwhile, her husband has also hired Wren, now an investigator, to find his wayward wife and bring her back home. Of course he has no intention to do so, but he takes the case and finds his quarry.
Reunited at last, with the terrible truth out, the three must work together to find a way to save Emilia, and themselves, from ruin and potential death. Along the way, the passions they felt for each other all those years ago are sparked like dry tinder until they can no longer be denied. Will they find a way to love each other for all time, or will the danger that stalks them have its way in the end?

This is the fifth book in the Theirs series, but it can be read as a standalone.