What versions of Windows do they have?
Are you trying to browse with all of them at the same time.
Who makes the NIC and is it an onboard Nic? or a pci Nic?
It sounds like the Nic needs replacing.
I have three computers in my home. I am having problems with the newest computer when connecting to the internet. The connection speed is less than half of the other two (19.2 kbps instead of 48 kbps) It is also showing receiving errors. Can anyone offer help to resolve this problem. Thanks