def get_trigrams(a_string):
    """
    returns the trigrams present in a_string.

    This will be empty for strings that are too short.  Repeated
    trigrams will be in the list multiple times.

    :param a_string:
    :return: 
    """
    return [a_string[i:i+3] for i in range(0, len(a_string)-2)]