def unpack3(var, struct_var, buff):
    """
    Create an unpack call on the ``struct.Struct`` object with the name
    ``struct_var``.

    :param var: variable the stores the result of unpack call, ``str``
    :param str struct_var: name of the struct variable used to unpack ``buff``
    :param buff: buffer that the unpack reads from, ``StringIO``
    """
    return '%s = %s.unpack(%s)' % (var, struct_var, buff)