(Lol didnt know the last update for the 3gs was 4.2.1.)
Yes so download the Redsnow v0.9.6 as i said before, but for the 4.2.1 version basically. If you download Green poison RC5 instead of Redsnow, it will be tethered, and therefore not unlocked. Although you can manually process the unlock, by going into cydia after breakingjail, and searching for
Rocky Racoon 4.2.1.