iCloud could be your culprit here. If you activated the photo stream and are taking pictures with your iPad while on the cellular connection, this pics will be uploaded into your photo stream over 3G. Same goes for some apps that keep some data in iCloud like Pages and a Numbers.
Go in your settings and make sure that iCloud sync can only occur over wifi and not 3G.
Another explanation could be that you may occasionally lose wifi connection without noticing it and surf the web over 3G.
I don't know if tethering is available on the iPad (3G), but my girlfriend had issues with her iPhone as her laptop would switch to the iPhones connection while plugged in instead of using the available wifi network. The issue is that the iPhone will drop the wifi connection and share it's 3G connection with the laptop when tethering is activated. But that may not apply on the iPad... So she ended up using lots of data while using her laptop while having her iPhone plugged in to sync her music.