Forgotten Promise by Mari Carr

She holds the key to their survival.

Kailani’s family have kept a secret for generations. She never thought it would put her in jeopardy until the night her phone rings and a stranger says, “Our society is under attack.”
With danger all around her, Kailani is forced into a deadly race across the country to find the others who share her secret. But she’s not going alone. Because before that call, she had an entirely different crisis on her hands–her arranged marriage to two men, including the man who broke her heart, Benjamin.
A centuries old secret becomes a race against time and the survival of the Trinity Masters hangs in the balance. Kailani, John, and Benjamin may not survive the mission. But even if they succeed, the fate of their trinity is at stake due to past mistakes and present tensions. And in the end, Kailani may seize the power to dissolve their marriage.