The assumption is that the voltages in a three-phase supply are balanced. However in real life there is always a unbalance. Power Quality Standards allow a voltage imbalance depending on every country or standard. During simulations and studies, the supply is assumed to have balanced output.

yes absoloutly in the mentioned system the voltage that apear across load is the phase voltage and that is balance this means that the nutral point of the star conected load is at the center of the star and not floated up or down hence the lines voltages will also be balance