The software shutdown is implemented by the LSb of the Shutdown open short register.
Hardware shutdown is implemented by the RSTN pin.
According to the datasheet all the RSTN lines are connected together for all chips.
Obviously, the Shutdown open short register is different for each chip...so you need to send 3 commands for 3 chips. 
The decision you have to make is whether you want to seperately Hard reset each AS1130 or whether you want to do them all at once. In the first case you need 3 dedicated pins, in the second only 1.
Or
You can leave the RSTN pins clear of the MCU and just use a button (this is what I have done)