Three Dark Crowns By Kendare Blake

When kingdom come, there will be one.
In every generation on the island of Fennbirn, a set of triplets is born—three queens, all equal heirs to the crown and each possessor of a coveted magic. Mirabella is a fierce elemental, able to spark hungry flames or vicious storms at the snap of her fingers. Katharine is a poisoner, one who can ingest the deadliest poisons without so much as a stomachache. Arsinoe, a naturalist, is said to have the ability to bloom the reddest rose and control the fiercest of lions.
But becoming the Queen Crowned isn’t solely a matter of royal birth. Each sister has to fight for it. And it’s not just a game of win or lose…it’s life or death. The night the sisters turn sixteen, the battle begins.
The last queen standing gets the crown.