Mapping Physical Networks on Hyper V

I have Hyper V 2008 7.6.10, and I have just created 5 virtual machines which I would like to provide 3 different networks to them. WAN, LAN and DMZ. 

The Hyper V original was installed with one NIC "LAN" but now I would like it to show the 3 of these NICs which are physically connected to the HYPER-V already. 

I have tried to browse to the Hyper V Virtual Network management console and click "Add External" then choose the Physical LAN but that would give me error setup installation all the time. 

I haven't done this before and I would appreciate how to do this! 

Thanks 

Mohammed JH