You shouldnt need to use softserial on Due, since you already have 4 serial ports!
To select which one you want to send the command to, simply add the number of the serial port after the "Serial" state.
For example:
Serial.begin(9600); would affect TX0/RX0, since you didnt write a number.
Serial1.print("Hello"); would affect TX1/RX1.
Serial2.read(); would affect TX2/RX2.
I hope this helps!
BR