Installing 3.1.2

[StellenboschStudent]

I had some problems with my phone so I rebooted and restored and in the process installed OS 3.1.3. I have a new 3GS so this means so jailbreaking for me... Is it possible to install OS 3.1.2 manually? I have a MacBook and downloaded the 3.1.2 file, if I click on it, iTunes opens up but nothing happens. Is there another method to install 3.1.2?

[Gothan]

usually when you "shift - left click" on a pc, you can select the firmware to install. But I am still on fw 3, so a lot might have changed hahahahahahahaha