3天前：取負值 

  Calendar calendar1 = Calendar.getInstance();
  SimpleDateFormat sdf1 = new SimpleDateFormat("yyyyMMdd");
  calendar1.add(Calendar.DATE, -3);
  String three_days_ago = sdf1.format(calendar1.getTime());
  System.out.println(three_days_ago);
  

同理，3天后，取正值即可：
  Calendar calendar2 = Calendar.getInstance();
  SimpleDateFormat sdf2 = new SimpleDateFormat("yyyyMMdd");
  calendar2.add(Calendar.DATE, 3);
  String three_days_after = sdf2.format(calendar2.getTime());
  System.out.println(three_days_after);