Diablo 3 is an account-based game, in that you need to register a CD key to an account to play it, regardless of if you have a physical disc. As a result, having the game does not directly give you access to the game.
While never a guarantee, its always an option to nicely ask your local internet cafe to put an up-to-date version of the game on a USB flash drive.
As you still have to pay for the game in order to play it, they are not as worried about copyright infringement, if that is an issue that concerns them at all.
Never a guarantee, and this depends entirely on your local internet cafe's manner and game availability, but if they oblige you can obtain an up to date version without any actual internet overhead.
This is how I often keep my Blizzard games up to date.