First of all, your provider probably prohibits the use of multiple computers over 1 cable connection.
If you use 1 connection with 3 compu’s only one of them (the one with the cablemodem) will actually download the files. If the file was requested by antother compu on your LAN, it will be passed to that compu, BUT it is still the compu with cable modem making the request and thus generating the traffic.
So you will be billed for everything you download from the internet over the cable connection, no matter what compu you used to download it. 2 GB is maximum, so if you have 3 compus connected, you’ll reach it 3 times as fast