code: public MailContent row(String firstCell,
                             String secondCell,
                             String thirdCell) {
          return tag("tr").cell(firstCell).cell(secondCell).cell(thirdCell).end();
      }
docs: Adds a new row with three columns.
  
  @param firstCell  the first cell text content
  @param secondCell the second cell text content
  @param thirdCell  the third cell text content
  @return the changed mail content object