If you look at the current flowing through the coil connected between A and C phases, you will see that this is strictly single phase. There can be no phase shift from one end of a transformer winding to the other end(even if there is a center-tap).
The normal 120/240 single phase calculations apply for any load connected A-N, N-C.
I=kVA/E, use L-L kVA if using L-L voltage
Normal three phase calculations can be used for any loads connected A-B-C.
I=kVA/(E*1.73), use L-L kVA if using L-L voltage
Now just add up the line currents.
