If three winding transformer is loaded differently in two secondaries, the unregulated secondary winding voltages cannot be managed with OLTC.
If, both PT and CT of both secondaries are connected to AVR, and AVR is programmed such that it should choose optimum tap position for both secondary windings regulation. If there is huge load difference in both secondaries and optimum tap position cannot be choosen, then AVR should regulate preferenced secondary winding voltage.