There are several ways to connect your computers:
1. Create an "ad-hoc" connection, the downside to this however, is that you will not be able to connect to the internet unless one of them is physically plugged in to your modem, and the connection is shared.
2. Get a ethernet switch for 20 dollars and physically hook up the three computers together via ethernet cables
3. Buy a router, and hook it up as above (2)
4. Buy a wireless router, and connect all three computers to the same network
*wifi is essentially the same thing as wireless internet, actually, it has gotten to the point where it IS the same thing
INSTRUCTIONS FOR SETTING UP THE SHARE
If you need help/run into problems feel free to shoot me a PM