Susan Kaplan’s 2004 documentary about a Manhattan threesome ought to be more engaging. Two young men fall in love and move in together, but they both also enjoy women. So after a few unsuccessful attempts, they add a woman to their relationship, Samantha, who eventually has a child by each. They all claim it’s not mostly about sex, but we learn little about how they negotiate their relationships over 13 years and even less about their careers or other interests. As their desires become more conventional and self-centered, fissures develop. Yawn. And Kaplan’s decision to violate documentary principles by using songs to “narrate” some sections is simply irritating. 95 min.