as far as I know free codes does not unlock your phone, so even if you would find a unlock code or whatever you'd have to spend some money.
This site has a nearly perfect with google checkout and they were recommended highly in the cell phone forums.
http://www.unlock-zone.com
I used them to unlock my cell phone. contacted them and within 30 mins i had my unlock code and my phone was unlocked.
Hope this helps.
Your only option is to call Three to see if they can give you an unlock code. I think the only way to do that is via Flash. Meaning you will need to take it in and purchase a service to flash unlock your phone so that the phone will become unlocked. Sorry.