will need to have a wireless network adapter fitted, and you'll need to buy a wireless router. Most of these have a broadband modem already built in.
The third computer can work without a wireless adapter, as long as you can connect it to the router via an ethernet cable, otherwise it, too, will need an adapter. Adapters can either be fitted internally (in a spare PCI slot) or can be external, USB adapters. These are a good choice if you're not comfortable with opening up your computer's case.
It all works like this:
1. The new wireless router is connected to a power supply, and to the ADSL telephone line.
2. You need to connect a computer (any one) to the router for the intitial setup process, and you do this using the ethernet cable that will be supplied with the router. During the process you'll access the router's configuration interface via your web browser, although you won't at that stage be connected to the internet. The process involves entering your ISP login details, and securing your new network by entering an encrypted password. This ensures that unauthorised people in the vicinity cannot use your wireless broadband connection without your knowledge. The information is saved to the router's firmware, and after that the router will make the connection automatically, each time you switch it on.
3. You'll need to install wireless network adapters on any computer that requires internet access - making sure that you install the supplied software before you connect the actual adapter.
4. At this stage you can disconnect the cable between the setup PC and the router, and install a wireless adapter on that machine.... or leave it permanently connected via the cable, in which case it will have internet access that way.
That's it - each machine will have access to the internet as long as the router is turned on. It isn't necessary to have the other machines running for that to happen. It isn't necessary (or desirable) to leave the router permanently switched on and connected to the internet - you can turn it off at night, or when you're out. It will automatically establish a connection for you when you switch it on again.