Why can't I update my device?
May be I am too easy with you. 
But just for the fact of helping, were you able to access internet through your mobile? 
If the answer is 'Yes' then there must be a bug with your OS. (There are possibilities for that in one or two devices)
My suggestion is to use 'Kies' from a PC.
Last week only I got to know even GT-I9300 devices are network locked in some countries. A friend of mine's GT-I9100 started asking for Network Unlock code all of a sudden. After a master reset, we noticed some new apps from Vodafone and the default language turned into Spanish or something. 
So I think your GS3 might be trying to access its original operator. 
Or may be some other reason. 
Just try my suggestions if you didn't do already.