Any time a 3G radio is on it tries to connect. It uses a huge amount of energy trying to find a connection. So if there is no service in your location, the 3G radio will eat up the battery in short order. If the service keeps switching between 3G and a slower connection like Edge it will burn up a lot of energy too. The same goes for any 3G radio whether in a phone or another device. So keep 3G turned off when you are out of a service area. Actually, I keep 3G off nearly always as it is seldom used in my case, and I keep WiFi off when I am no using it.
Currently: Kindle Touch, iPhone 5, and iPad 3.