It may also be that you are out of IPs. For instance, if you have cable modem or DSL service, most providers allow only 1 IP (and you 'pay' for additional IP addresses.) You say that you are using a 'hub' for your desktops (which is different from a 'router'), so in theory you are using 3 different IP addresses depending on what the hub is connected to.
Are you plugging the laptop into the same hub? If so, you should make sure you arent plugging it into an 'uplink' port.
Have you tried unplugging the cable from one of the desktops, and plugging laptop in that port to see if it works?
Have you tried a different CAT cable? (it may be that you grabbed an old/faulty or maybe even a crossover cable by accident.)
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