Hi,
There are many ways to SIM free a Iphone3gs like calling the service provider and requesting him to unlock your Iphone,but this may take some time may be days,weeks or even months to proceed .The second method is by taking your phone to a near by shop and getting it unlocked,but in some cases the phone may get bricked or even get hard locked.The third way is by using the third party unlocking service,one of the third party unlocking service I know is remote unlocking service .Which I would say is the best and reliable way to unlock a Iphone from Its carrier,If you are interested then you can get this remote unlocking service from the site You must login or register to view this content. .You can avail the service by providing some basic information about the phone like the country and the network to which the phone is locked to,after providing this information you can avail the service from the site to unlock your Iphone to use it with any other GSM network of your choice.

You will need to buy unlock service from ebay. It's very competitive on ebay should be around 2-3$.