that they're not in halls, but in a private house/flat. You mustn't set up wireless networks in halls of residence without permission - for obvious reasons.
You'll need each machine to be equipped with a wireless network card, or an external USB wireless network adapter.
Then you need a wireless network access point - a base station in effect. This can be connected directly to one of the computers.That's it as far as hardware goes. In an ideal world all the machines will be running WinXP, which makes setting up the network a tad easier, but it will work quite happily with a mixture of Operating systems.
You'll need to think carefully about the siting of the computers - wireless networks are good, but over three floors you will definitely experience some dead spots.