1. Jailbreak the phone and install TetherMe. I will be happy to provide some links to help you with this if you choose to go this route. This is what I did recently, and it's really no big deal. I can even suggest a way to get the TetherMe app for free.
2. Try a "hot" T-Mobile SIM swap. The procedure quickly is get your hands on a T-Mobile SIM (I've read that it doesn't matter whether it's an active or inactive one), remove your ST SIM, insert the T-Mobile SIM, give the phone time to acclimate to the new SIM (may take several minutes and you need to be in an area with T-Mobile coverage), go to Settings>General>Network and Cellular Data Network should be available, enter the correct settings for MMS, remove the T-Mobile SIM, reinstall your ST SIM, and you should have working MMS. I was going to either order a T-Mobile SIM off eBay or go in a T-Mobile store to test this out, but I decided to jailbreak instead. You will see most reporting that this works for the 4/4S, but not the 3GS. I have seen reports that it will work for an unlocked 3GS like yours and mine, but I can't say for sure.
3. "Modify the preferences.plist and com.apple.mms_override.plist files in your iPhone backup and restore to the phone using iBackupBot". Here is a link to a recent thread that explains this http://www.howardforums.com/showthre...k-no-jailbreak