And your mobile data is set on as well as 3G mode enabled?
If so, check APN settings. It doesn't matter that you've not changed anything, check them anyway. We need to eliminate simple things like this before looking for other causes.
You can test whether you are connecting to 3g by doing the following: open your phone dialer and dial *#*#4636#*#*. You don't need to hit the call button, this should put you straight through to the "Testing" menu (though I think Three UK remove this in their software). Select Phone Information and see what it says for "Network type", "Preferred network type" and "GPRS service".