Ya sure you can use t-mobile prepaid sim in Iphone 3gs only if it is unlocked one ..Else if the iphone 3gs is locked to some other network then until you unlock it you can't use it with t-mobile sim ..You can easily unlock your iphone 3gs from network lock with the help of providers like Theunlockarena.com here they will unlock your iphone 3gs using remote unlocking service for affordable cost ..After unlocking your iphone you can freely use it with any GSM sim ...
