This sounds more like a hardware problem, or maybe faulty share settings.
First, I would see if there is a diagnostic for the network card in computer Three.
If it is a motherboard ethernet port, install a seperate PCI card and disable onboard.
Also, test the cable going to computer Three with a cable tester.
Another thing to look at is where the share is defined.
if computer Three is accessing files from computer One,
be sure it is connecting directly to computer One, not to a share from Computer Two that points to Computer One. I’ve seen this once before.
In your case, there might be a conflict or improper configuration with the network settings. You need to carefully check the network configuration of that particular computer unit and try to set it the same with other computer units.
It is really possible that a single computer could cause the entire network of computers to slow down. This situation could be caused by a virus attacking every computer unit within the network, make sure that every computer unit within the network has its own antivirus software installed.