assuming one of 3 computer hae 2 NICs, yes, you can make the computer with 2 NICs as a router. quoted from
http://www.howtonetworking.com/Networking/routing.htm.
How to Configure Windows server to Be a Router
RRAS is installed under w2k server by default, but not activated. To setup Windows 2000 as a router for a LAN, you need to two network adapters.
To setup Windows 2000/2003 as a router for a LAN, you need to two network adapters. To enable LAN routing. go to Administrative Tools>Routing and Remote Access>Action>Configure and Enable Routing and Remote Access, and then complete the wizard. Right-click the server for which you want to enable routing, and then click Properties>General>Router, check Local area network (LAN) routing only, and then click OK.