Download the app Advanced Task Killer:https://play.google.com/store/apps/details?id=com.rechild.advancedtaskkiller&hl=en_GB
You'll see which apps open themselves up and transfer data without your knowledge. For example, I rebooted my phone this morning, killed some apps, yet an hour later the following have started without my say so:
All of which use 3G if allowed.
Personally I keep 3G off unless I need to search for something. It saves power and data.
GPS only uses data if you allow Google to track your location via 3G to aid accuracy.