It it not possible to jailbreak a 3GS already on 3.1.3. If you have your 3.1.2 SHSH saved (either on Saurik's server via Cydia or locally via Umbrella) then you can downgrade to 3.1.2 and jailbreak. Otherwise you will not be able to jailbreak for the foreseeable future.
There are several threads on the first page of this very forum which would have answered your question. In fact, there is one entitled "Jailbreaking a 3GS 3.1.3 using Windows" just four threads below yours.