I am thinking of adding a third nic card to the system, I am hoping this is
going to be able to separate one network from another,

I wanna offer a public hotspot, but don't want the hotspot to be able to get
to my main network

Main network            on lan ip of 192.xxx.xxx.xxx via dhcp on the "LAN"
nic card of monowall
Public hotspot          on LAN ip of 10.xxx.xxx.xxx via  "public ip
software" on the third nic card


Http://majesticsoundsonline.com/ac/public.jpg
<Http://majesticsoundsonline.com/ac/public.jpg> 

This is hat I had in mind I have a diagram 



this is the setup I am thinking of

would this be poss. or safe