Sorry, crossed posts! The URL in your post is a normal 'http' one but the relevant page uses a secure 'https' address. Try this: https://www.three.co.uk/Support/Device_Unlock
If you are still under contract, the phone operator will not let you unlock it. If you are out of contract, you should ask Three for the PAC code, and they will send you a link, from which you request unlocking. It can take several days.
is it virgin media?
as i tried to go on the google search page,clicked the three site
then to the three store and i still get the message
You don't have permission to access "http://www.three.co.uk/store?" on this server.