yes it is possible to jailbreak and unlock iphone 3gs.
jailbreaking and unlocking an iphone is born together steps for iphone. once you jailbreak your iphone you need to unlock it. jailbreaking your iphone can be done only through your current software version which your phone has. it may be of redsnow/cydia/yellowsnow.. what ever it may be the following are the steps for jailbreaking. this link
http://mobiputing.com/2011/12/how-to-jailbreak-an-iphone-ipod-touch-or-ipad-with-ios-5-0-1-untethered/ shows you how to jailbreak the iphone using redsnow. the same steps follows to other softwares too only thing is the software differs. after jailbreaking your phone you need to unlock it. you can unlock it using softwares or some other methods. if you ask me i will say remote unlocking is the best way to unlock iphones. to get this service you can contact
Simpleunlocking.com or some other providers who provides this service.
