def UNKNOWN_WORKFLOW(project_id, branch_id, workflow_id):
    """Error message for requests that access workflows.

    Parameters
    ----------
    project_id: string
        Unique project identifier.
    branch_id: string
        Unique branch identifier.
    workflow_id: string
        Unique workflow identifier.

    Returns
    -------
    string
    """
    msg = "unknown workflow '{}', branch '{}', or project '{}'"
    return msg.format(workflow_id, branch_id, project_id)