Once Upon A Quest by Joanna Mazurkiewicz

Meet Catherine of the Southern Kingdom—a foul-mouthed, unladylike Princess who never anticipated three dhampir Princes aiding in her quest to save her sister’s life.
Catherine is next in line to become Queen of the Southern Kingdom; she’s a fighter not a leader. She’d much rather slay dragons alongside the royal guard than wear a crown.
During an outing in the forest, her sister Lucy is kidnapped by the Drakin Clan—cruel dragon shifters who’ve been terrorising surrounding kingdoms for decades.

At the request of the king, and Catherine’s only option, she must strike a deal with the three princes of the Western Kingdom.
The wickedly handsome dhampir brothers insist upon her choosing one of them as her mate and future king to rule alongside her in return for their aid in rescuing Lucy.
These men will fight for her, drive her insane and compete for her affection. Will she be able to resist their forward affections or finally open her heart to love?

A dangerous quest, a feisty, badass princess with a destiny who must keep her cool around three insanely inappropriate dhampir Princes.