Ground Fault Question Regarding a 3 Phase Circuit
A current transformer is just like a regular transformer. The ratio of current in the primary to current in the secondary is just the turns ratio. In this case 1000. So for a current of 1A in the primary, you will get 1mA in the secondary. You need to drop this across a known resistance to get a voltage readout.

In the three phase circuit you have shown, the secondary currents add and give you just what you want, the total current through all three lines, which should add to zero if there is no ground fault.

The Allegro ACS714 in the secondary will give you 180 ~ 190 mV per Amp in the secondary, that is 180 ~ 190 uV per primary Amp. I do not think that is what you want. You will want something to give you 1 Volt per mA. Maybe just a 1K resistor.