i have to ask, as The Google yields inconclusive results. What is Sage and how do you want to use it?
As far as networking goes, there are several ways to get your locations connected. You will need Internet connections in all locations. However, from there, the options are varied depending on the level at which you wish to perform.
Windows has a built in remote desktop program. You could use that. One dedicated machine could allow the other two locations to connect to your application. The drawbacks are that RDP is not very secure and Windows (XP, Vista, 7), by default, is not designed to allow multiple connections. Even then, you have to have the Professional version of the OS. Any version lower than Pro does not allow you to receive RDP connections. You could hack the OS with a patch to allow RDP connections as well as concurrent connections for RDP. See
http://sig9.com/articles/concurrent-remote-desktop for information.
Probably the best option is to use a VPN/Firewall box. This will allow you to configure each location with the information necessary to connect to the other locations as if they were all in the same room/building. It would also provide you a reasonably secure connection. You would configure the boxes to connect to each other and route traffic required to share resources. A security key would be used to transfer information from location to location via the Internets. Drawbacks are: some boxes are pricey, the cheaper ones work well but will vary in quality of the build, configuration will vary with the price of the device you purchase.
The third way you could do this is to have a dedicated Server machine in one office and configure VPN connection from each location to the Server. The Server would run the application and dish out information to the locations as requested. Drawbacks: Servers can be pricey and configuration can be mildly frustrating if you do not know what you're doing. You can technically make any machine a Server with the installation of a Server OS. The Server OS itself is the majority of the cost of this application. There are different Server OS levels for different levels of business. See
http://www.microsoft.com/sbs/en/us/pricing.aspx
When planning a network, be sure to include growth options. You don't want to put out money now and discover that a year from now you have to drop more cash to get things where you need them to be. Example: Purchase Small Business Server and realize that you really need the options provided in a higher level software.
TL;DR Use carrier pigeons.
