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you'll need, at the very least, a network between the three machines. this mean you need a NIC (network interface card) for each computer. you can pick up some wireless network adapters at any computer store, i reccomend linksys.
ideally, the best thing to do would be to set up a network using a wireless router/gateway and conencting the modem to that. then the pcs would connect to the router via a wireless network and you'd have a solid network.
the other option is to let windows manage the sharing if internet from one of the computers. this eliminates the need to purchase a router but is far less secure and you can't access the internet if the host pc is shut down. there are additional drawbacks as well.