Their romance is a messy eternal triangle.
eternal triangle
idiomA relationship involving three persons (usually two women and one man or two men and one woman) among whom there are conflicting and competing attachments of a romantic or emotional nature.
The novel describes an eternal triangle between two best friends and the woman they both love.
Caught in a painful eternal triangle, the protagonist struggled to choose between his long-term partner and the person who had recently captured his heart.