It won't get Siri. Even on iOS5 only the 4S supports Siri.

As far as I'm aware this is a deliberate disabling thing as the original Siri app worked on earlier models (maybe not the 3GS but the 4).

No Siri (still), and I'm fairly sure I heard no 3D maps.

I'd be very surprised if iOS 6 included Siri for the 3GS (or even the 4, for that matter).

This is just a guess, but since the iPhone 4 currently doesn't support Siri, I doubt the 3GS would.