Arduino(s) not resetting on Serial connection lost / re-established - why?
What you have done is to disable the "HUPCL" signal that is sent when the port is opened/closed.  

You can turn the signal back on with:

$ stty -F /dev/ttyACM3 hupcl


And you can turn it off again with:

$ stty -F /dev/ttyACM3 -hupcl


HUPCL means "HangUP on CLose".