You could store the files on one PC then share the resource for the other two. However, this requires that this one PC be turned on for the other two to access it. Another low cost solution is to purchase a network capable hard drive, rather than a dedicated server. These storage solutions can hook up to the network and allows you to add multiple hard drives to create a redudant, fault tolerant NAS system.
With regard to remote access, you could install an internet router that provides a VPN service. This will allow one or more users to connect from outside the network and access internal resources. Or if all you need is a remote desktop for one PC, you could simply create a port forwarding rule on the internet router. Open port 3389 on the router and map that to the private IP/3389 for the target computer. Then on the remote computer, just start the RDP application and type in the public IP or better yet, fully qualified host name that maps to the public IP address of the router.
JorgeM
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