Help on (another) triphasic exercise
This is just a partial answer that may help.

The diagram seems to me to be very badly drawn or the system is unbalanced.



Figure 1. VAB, VBC and VCA redrawn in conventional fashion for a balanced system.

If the system is balanced then UA, UB and UC would be equal length and UAB, etc., would be &radic;3 times the length of UA. As drawn they don't translate properly. Again on a balanced system UC would be at right angles to UAB.

Good luck.