See if your network do the International 4G version (NOT the poor American version with dual cores!) and if they do, get it exchanged if you've had it less than a week or two
oh and don't fret too much, the i9300 3G S3 is more than capable of doing anything you could want it to! It's ridiculously fast, even on apps that aren't optimized to use all of it's power!
Oh right ok, i don't suppose you know if Three stock the i9305 do you? Im a bit disppointed to find out theres a better version with 4G capability available.
It's marginally better, same processor, just a bit more RAM and 4G connectivity - 1Gb has not been an issue for me, I can play games like NOVA3 and The Dark Knight Rises, Men in Black etc and 4G, well EE will only have it in 16 UK cities by the end of the year, and what sort signal strength will it be in those cities ?