Basically, it depends on the modulation technique you are using as well as the control technique. for example, if you are using simple current hysteresis controller,  the voltage of each phase is adapted to produce sinusoidal current according to your reference. but if you use three-phase voltage reference with PWM modulation, you are controlling the voltage hence the current is arbitrary depending on the loading condition. 

can you first state the application (motor drive or grid-connected inverters?) second, can you state the control method or what do you want to control?

about your second question, the inverter has no firing angle, it is controlled by modulation like PWM or SVM, hence we are not sealing here with firing angle but the switching frequency. 

I hope I understood your question.

Best Regards

I suggest you to read about series active filters and search about unbalance compensation to understand how its possible to control different phases of inverter.

Regards,

Dear Hany and Davood, 

let me frame my question this way. Imagine that the voltage at 2 phases of the bus that inverter is connected is fine, and the other one is below an acceptable threshold, and we want to inject reactive power to that phase. Is it possible? can a 3 phase inverter correct that voltage without messing with the other two?

Now is clear. you can use Converter with Dual Vector Control which can deal with the negative sequence component. so even if you have only one phase dip, it is possible. I attached a very interesting paper for you.

M. Bongiorno and J. Svensson, "Voltage Dip Mitigation Using Shunt-Connected Voltage Source Converter," in IEEE Transactions on Power Electronics, vol. 22, no. 5, pp. 1867-1874, Sept. 2007.

doi: 10.1109/TPEL.2007.904243