The triangular structure of desire names the insight that erotic longing is never a simple dyad between subject and object but always involves a third term — a gap, a rival, an interposed medium, or an idealised image — that is constitutive of desire itself rather than merely incidental to it. Within the depth-psychology corpus the concept is approached from several convergent angles. Anne Carson, drawing on archaic Greek lyric, demonstrates that the triangle lover–beloved–space-between is the irreducible geometry of eros: desire requires lack, and lack requires distance, which means that any representation of desire structurally introduces a third pole. James Hillman, working within archetypal psychology, argues that the triangle is not a symptom to be dissolved (as Oedipal readings would have it) but an objective necessity of soul-making: jealousy, impossible love, and third-party fantasies serve the creative life of the psyche. Jacques Lacan approaches the same configuration through the logic of the partial object and the phantasy formula: desire is always desire of the Other’s desire, installing an irreducible third locus — the place of the dead father, the symbolic phallus — around which the subject’s wanting circulates. Across these voices a shared tension persists: whether the third term is fundamentally generative (Carson, Hillman) or symptomatic of a structural impossibility that can never be resolved (Lacan). The concept matters because it reframes jealousy, rivalry, transference, and narrative itself as formal properties of desire rather than accidents of circumstance.