TANGLED UP IN BLUE By Larry Duplechan (St. Martin's: $8.95). The well-rounded characters and agreeable style of Larry Duplechan's previous novel, "Blackbird," are sadly absent from this turgid potboiler. A lopsided romantic triangle involving a gay man, a heterosexual woman and her husband (who had an affair with the gay man but isn't really gay) unleashes numerous and predictable complications, which Duplechan recounts with the leaden superficiality of a bad soap opera. The story hits bottom around Page 68, when the gay man is diagnosed as having AIDS-Related Complex (ARC) by a doctor who dismisses the idea of HIV testing and provides no medication or treatment. But that still leaves nearly 200 pages of stormy break-ups, tearful reconciliations and flaccid declarations of affection. Slogging through this bilge approaches an act of masochism, and few readers will care to subject themselves to it.