def build_key_name(app_name, os_name, file_name):
    """ 
    Creates key using app name, os and filename

    :param app_name: app name
    :param os_name: OS the app is written for
    :param filename: the name of the file
    :return: S3 bucket key for given app/os/filename combination
    """
    return (app_name.replace(" ", "").lower() + "/" +
            os_name.replace(" ", "").lower() + "/" + file_name)