Unfortunately no. Siri will not extend to any device that it's not already on, the 4S. So, the 3GS and 4 will not have added Siri funtionality w/ iOS 6.
the new ipod touch will get siri the new ipad will get siri and of course the 4S already has it and naturally so will the iphone 5, any other devices such as the 3GS wont get it and not even the 4 will get it as Thomas pointed out.. But youre not missing much as siri is something you dont ever use.. well I never dont any way