Hi goldkwang
When using a 3phase metering chip like ADE7878 for 3P3W delta configuration VRMS measurements, you will need to assume a common leg for the three voltage measurements, as all the single ended voltages VA,VB and VC are in reference to VN. For the first case, tie the earth ground to neutral leg ( if it is possible in your application) and you can measure all three phase voltages with respect to that earth ground. For the second case, where you need to measure line to line voltages, if you can assume one phase as the reference, then kindly follow the configuration as illustrated in page 15 of the App Note. By making the assumption that the loads are balanced, you can achieve this. Does this work for you? If not, kindly provide some system level diagram of your application setup and we can discuss about other possible options that you might have.
Regards
hmani