and 3 Ethernet cables.
the only other way is the PC can do wireless, and you witlessly link the PC and ICS [internet connection sharing if you have window,s if you have unix you need router software] the xbox via ethernet to the PC. The computer must be on to use the internet on the xbox on that design. You also have to go into windows and tell it to share from wireless to the Ethernet and set up the IP system.
For clarity the later option is a horrible idea, but if you have to do it that way, then its possible.
A router and more cables is the better solution by far.http://windows.microsoft.com/en-US/windows-vista/Using-ICS-Internet-Connection-Sharing