netbui is not routable, but it shouldnt matter since you are on same segment. Check your windows firewall to make sure its allowing netbui. Honestly you shouldnt be sharing on netbui anymore anyway, use IP. And furthermore you shouldnt be sharing files on an open network like that, its total suicide. TOTAL.
Get a Zyxel as I said, then share over IP and no worries.
Now, in what way is yoru filesharing not working? Can you not see the other computers in the network neighborhood? Its an issue since your computers are connected to the internet, when they ask which computer is the masterbrowser, its broadcast across your whole segment which is your 3 comps, your neighbors system, and probably a few hundred other systems. So if any has been set as master browser and responds to your computers as the master browser, then your computers will be trying to work with that foreign computer to set up file/printer sharing. Its bad on so many levels. SO MANY LEVELS.
Get a Zyxel 334WT.
if your deadset on this I would say, as a test assign static IP addresses to each of your comptuers, then unplug the cable modem or whatever connects you to the internet, then reboot each computer. Then see what happens. Definitely uninstall/disable netbui as its just not needed and can only confuse things.