I'm a little unclear on exactly what you're asking, but you shouldn't need any kind of server (unless you already have a server at the office that you could add a task to) Just use a good routers with VPN functionality at all three locations and you can connect them into one virtual network (look into dd-wrt, it's a third-party linux based firmware you can install on many residential routers to give them commercial grade features such as VPN)

You can also install a VPN program (I've used logmein Hamachi in the past, but I'm sure there's a better alternative) to connect them. You'd have to install a small program on each computer, provide it with the information for the other systems you want it to network to, and it will create a virtual network over the internet allowing you to share network resources whether the computers are locally attached or not.

If this isn't what you were looking for please try to explain what you want in more detail, and describe the current network configuration.