Accessing Program On Another PC

I use Sage to update my stock on a PC in my back room.I want to access that PC and open the sage program from another 2 PC's, add data, save etc.I thought a simple network would do it but not quite. I have three PC's in different parts of the building.I can see all three PC's and share files, printers etc, but not sure how to execute and run the program through the network on the back PC.I didn't want to set-up a server but in a way this looks like what I need to do.How much more after what i've set-up as a network with all sharing and internet connections etc do i need to do?I'm thinking if i use the PC the program is on as the server, how would i be able to execute the program from another PC in the network.