def process_string(instring, year, month, day):
    """
    Process a string replacing placeholder occurrences of YYYY with year, MMMM with month and DDDD with day.

    :param instring: string to be converted
    :param year: year to replace in input string
    :param month: month to replace in input string
    :param day: day to replace in input string
    :return: string with filled placeholders.
    """
    sy = "{:04}".format(year)
    sm = "{:02}".format(month)
    sd = "{:02}".format(day)

    outstring = instring.replace('YYYY', sy)
    outstring = outstring.replace('MMMM', sm)
    outstring = outstring.replace('DDDD', sd)

    return outstring