def get_nth_subkey_letters(nth: int, key_length: int, message: str) -> str:
    """Return every nth letter in the message for each set of key_length letters in the message.

    Args:
        nth (int): Index to return for every key_length set of letters.
        key_length (int): Length of the key.
        message (str): The message

    Returns:
        str: The string of the nth letters
    """
    # Remove non-valid characters from the message
    # message = VALID_CHARACTERS_PATTERN.sub("", message)

    i = nth - 1
    letters = []
    while i < len(message):
        letters.append(message[i])
        i += key_length

    return "".join(letters)