I've gotten current measurements for all 3 phases before though. They are just sine waves. And not messy sine waves either. Actual clean looking sine waves. Then, the current coming from the capacitor is
currentCap = (duty1*current1 + duty2*current2 + duty3*current3)/3. But that's an average current I guess. But if we pretend that there's no help coming from the batteries, we could look at what happens for one period. We could assume that the current is constant the whole time, and figure out the voltage drop from the current and capacitance.
