Crew Headset Configuration?

Ok guys, I have a problem that I am sure many of you have solved.

We are currently building a 727 home cockpit and are in the process of configuring the headsets that will be worn by the crew for communication with each other and also communication with ATC on Vatsim.

So here is the problem, in real aircraft all crew members can hear each other speak through their own headsets and can communicate with ATC simultaneously with the press of a button.

I can configure it to the point of connecting all three headsets together and being able to transmit and receive radio transmissions via any headset, BUT how can I also configure the system so that all crew members can be heard through each others headsets as well as their own? (Windows built-in microphone monitoring feature will not work as it has about a half second delay.) :roll:

http://www.ruscool.co.nz/cp_intercom.html