I have an AT&T iPhone 4S 32GB running iOS 6 Beta 4 and I can confirm that the 3G toggle is not present. There is a toggle for cellular data but specifically 3G it is not, so its all or nothing. This may have something to do with the fact that the iPhone 4S on AT&T is running "4G" so having a 3G toggle would seem to be contradictory. For anyone that may be confused here reading this the iPhone 4S "4G" on AT&T is not what many consider to be REAL 4G as in LTE 4G. The iPhone 4S is HSPA+ which AT&T markets as "4G" but really its more like 3.5G since is only 2-3X faster than regular HSPA 3G. 4G LTE however, is MUCH faster, often 10X faster and potentially more since this technology has the room to expand. LTE 4G is expected to be used in the iPhone 5.
So to re-cap, AT&T iPhone 4S does not have a 3G toggle. Most likely due to the fact that it is supposed to be "4G" so a 3G toggle would contradict that point so it has an all or nothing cellular data toggle.