You will not want to use substream to send to another application - the video quality will not be as good and it will miss-read plates.
I believe you can stream the mainstream to 3 devices at the same time.
Going off your other thread, I encourage you to try the trial that Plate Recognizer offers - simply create an account and they give you a token number and you put that into
Blue Iris and you start getting plates with the plate number placed on the alert video and in the BI log.
Actually try Plate Recognizer and OpenALPR trials and see which one you like better - some here like OpenALPR and others like Plate Recognizer.