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They are only secure if NONE of them have Internet access.

If this is so then the inexpensive, and very secure way is to network them together using network cables joining at a “Hub”(simply put – a dumb router) or a router.
The router is easier, but if it has wireless, you will have to disable it(wireless) to prevent access from others.

You may have to take some time to configure firewall and or antivirus software to allow access to all PC’s on the network. You can actually turn off all firewall software since you will have a secure network. But I would advise leaving Anti-Virus running.(it can also prevent network access)

For each pc,enter your control panel and then network connections.
you can find the “set up a home or small office”option.Click it and follow the wizard to enable files and printer sharing.

The best way to network all 3 computers is to get a 4-port switch and 3 network (RJ45/Cat5) cables. These items are fairly inexpensive. Once it’s all connected, all you need to do is pick on of the computers and set up a shared folder or drive.