If you haven’t built or packaged your game and added in code to support this then you will have to do it manually through the console. Start one game up using the Dedicated server option in the editor … then start the other 2 clients up … press the tilde key “~” and enter the command to connect to your IP Address of your server.
I am not in front of my machine to test this … but I understand that this is suppossed to be how it works. Anyone feel free to jump in here and help out. 8-}
This link may help: Networked Play In Editor