This is for people who are already using Windows Phone 8 devices. Is it possible to specify your mobile network connection speed to be 2G only or 3G only? The only available options I had on my Lumia 800 were 2G only or 2G/3G. It was annoying not being able to set 3G only.
The 2G/3G option is a failsafe. When in the setting the phone will always use 3G, unless it is not available and then it will fallback on 2G. 2G and 3G are two completely separate bands and not all areas have the 3G band. This will prevent a disconnection from data and save battery life.
That said, different devices have different radios and will give you different options. It all depends on the carrier branding and the specific ROM setup for that carrier.