Do I need to jailbreak my Iphone 3gs in order to put a T-Mobile sim card in it?

Do I need to jailbreak my Iphone 3gs in order to put a T-Mobile sim card in it?

Yes and no. Your iphone needs to be unlock. Then you need to jailbreak it to receive mms or data.

Unlock.

Though, depending on which iOS you are running there may not be a free option to do so. Check chronicunlocks.com for how to do so.