The Trident Queen by Amy Sumida

The Three are gathering.

On the continent of Astari, the Zodiac Queendoms are run by twelve powerful women. Every hundred years, the Queens ascend to the constellations and new queens are chosen by the Gods. The chosen ones are marked by their Gods and receive divine tools called astrals to help them channel the magic of Astari into their queendoms. This period of rising queens is called the Dawning and sometimes it doesn’t go smoothly. This is one of those times.
In the island Queendom of Luhaera, Shasa Naehadrian is trying her best to live a simple life as a sea sponge farmer, but then the war over the next queen takes the lives of her parents and nearly hers as well. She is saved by a merman, one of the Children of Pisces, who tells her that he was sent by Pisces himself to protect her as his chosen queen. Mourning the loss of her parents, Shasa wants only one thing—vengeance—and she refuses to accept Pisces’ Mark or his astral—the Trident—until she has it. But the further she wanders from the path to the throne, the closer she gets, drawn to her fate by magic, her god, and the love of the three men destined to rule beside her.