There was some initial progress with dronecell, however due to the lack of popularity its no longer manufactured unfortunately
There are however many serail GSM shields that are compatible with arduino which could be used, Others have used a raspberry pi. I am just not sure of any good tutorial out there for setting it up with ardupilot/ autopilot.
Why do you specifically need to use 3G? no-one really does this as the 3DR 433mhz telemetry modules work really well for long ranges (2km+)