Did this work before or are you new to 48?
If it worked before and doesn't now then what changed?
If you are new then make sure that the internet settings are correctly configured.
Try the online configurator - there are a number of S3 (S III) models listed - choose the appropriate one that matches yours:
http://48months.tweakker.com/choose/Samsung
If that doesn't work then download the Tweakker APN app over wifi and run it. It normally works:
https://play.google.com/store/apps/details?id=com.tweakker
You also need to have data roaming enabled.
48 is a virtual carrier operating on the O2 network. Many users will see a roaming 'R' on the signal bars. This is normal and does't involve any additional cost or services issues.
Hope this helps.
