First things first... You and your 2 friends must collaborate toward network settings that are compatible. Then you can count on GIT working like a charm.
1) Make sure there are no IP conflicts between
any machine on all 3 networks.
To be honest, it's easiest to play behind NAT/router/firewall) where all machines have private addresses on the same subnet). If one or more of you are directly connected, I can't be much help because I don't have experience with that. You can find info in the forum, talking about connection without NAT/router/firewall. In that case, look for information about setting up a virtual loopback adapter. That will enable the private address.
edit: Forum Thread Link for information about setting up the virtual adapter for non-NAT clients.
/edit
2) Open/forward GIT connection ports on any firewalls. You'll be using udp ports 213, 214 & 215 (between all of you) to make a 3 way LAN. With GIT, you must connect to each other LAN on individual channels, and vice versa. Example: to represent your three physical networks (you and your two friends), I will use A, B & C. A & B talk GIT on port 213, A & C talk GIT on port 214, and B & C talk GIT on port 215. GIT tunnels packets along those communication channels according to its configuration. See?
This thread is talking about game/application ports to forward and instructions for port forwarding/virtual servers on various networking devices.
3) Once you're this far, make the GIT connections, then test them.
Enable ARP and ICMP on the Advanced Config screen. Ping the private addresses of your friends. Then, lookup and forward C&C protocol and ports in GIT configuration, and play away.