Can't boot tethered my 3GS goes into boot loop
If your 3GS is made prior week 27 20211 you can install the ipad baseband and then downgrade to 05.13.04 BB which can be unlocked with ultrasn0w when jailbroken.
Depending on which carrier the phone is locked to you can request a permanent IMEI unlock with the carrier or get a cheap IMEI unlock from the grey market online.
The jailbreak will be tethered, you have any (full) shsh/blobs for 6.0-6.1.2 youight be able to downgrade the firmware and get an untethered JB.