in 3 phase metering, it's desired to measure the voltage and currents of all 3 phases simultaneously. How is this synchronization done with these 3 separate ADCs?

The synchronization of all the ADC samples is possible anytime.

The ADE9113 are simultaneous sampling sigma delta ADCs sharing the same clock which gets passed along from one ADC to the other very quickly. So they sample at the same exact moment with all phases and all voltages of the system. We introduced counters inside these iso ADCs to synchronize the sample outputs on the MCU.