For more than a century, American parents have prodded their kids to drink three daily glasses of milk, but now the tide may be turning against this once seemingly essential beverage. A commentary published Monday questions the value of three servings of milk daily and whether the harm outweighs the benefits when people drink reduced-fat milk instead of whole milk. David Ludwig, HMS professor of pediatrics at Boston Children’s Hospital and Walter Willett, chair of the Department of Nutrition at HSPH, co-authored the commentary.