A gay, polyamorous California throuple made history in 2017 when they became the first family in the state to list three parents on a birth certificate. Their reproductive journey and legal battle to become fathers to Piper, now 3, is detailed in “Three Dads and a Baby” (out March 9, Cleis Press), written by one of her dads, Dr. Ian Jenkins. Jenkins and his partners, Jeremy Hodges and Dr. Alan Mayfield, don’t see their family — which now also includes their son, Parker, 1 — as unusual. “The fact that Piper has three parents is just not a big deal....