  For the two-user multiple-input multiple-output (MIMO) broadcast channel with
delayed channel state information at the transmitter (CSIT) and arbitrary
antenna configurations, all the degrees-of-freedom (DoF) regions are obtained.
However, for the three-user MIMO broadcast channel with delayed CSIT and
arbitrary antenna configurations, the DoF region of order-2 messages is still
unclear and only a partial achievable DoF region of order-1 messages is
obtained, where the order-2 messages and order-1 messages are desired by two
receivers and one receiver, respectively. In this paper, for the three-user
MIMO broadcast channel with delayed CSIT and arbitrary antenna configurations,
we first design transmission schemes for order-2 messages and order-1 messages.
Next, we propose to analyze the achievable DoF region of transmission scheme by
transformation approach. In particular, we transform the decoding condition of
transmission scheme w.r.t. phase duration into the achievable DoF region w.r.t.
achievable DoF, through achievable DoF tuple expression connecting phase
duration and achievable DoF. As a result, the DoF region of order-2 messages is
characterized and an achievable DoF region of order-1 messages is completely
expressed. Besides, for order-1 messages, we derive the sufficient condition,
under which the proposed achievable DoF region is the DoF region.
