try a test.

Disconnect the other two computers from the internet and hard wire this problem one to the internet, (direct cable connection to router or modem). Does the internet speed improve?

If not, do a speed test at CNET's Bandwidth Meter while this computer is still the only one connected;

If you install a different browser on this system, does that improve internet speed?

The reason for the tests? To find out if this is a bandwidth problem or a problem with the computer itself.

Tell us more about the setup. What type of internet connection do you have, eg dial-up, broadband, (what speed)?

How are the 3 computers connected, router, router hub, wireless, modem?