http://www.three.co.uk/_standalone/L...=1214305748126
Probably page 8, pay as you go mobile broadband addons.
You will probably find that it is a better deal to go on eBay and buy a 3GB SIM there. Maybe even consider EE instead - you get 6GB for a lower price on eBay than 3GB with 3, and you have the benefit of 4G too, if it's available. Another downside is that addons bought afterwards have a 30 day expiry, but the initial data allowance is typically 90 days
Unfortunately none of the networks have very attractive mobile broadband options. If you can do a monthly contract (even a 1 month one) it opens up better value. I think 3 have a 15GB plan.
When the initial 3GB runs out, it will just stop working. You will get a webpage pop up saying so and asking if you'd like to buy a new addon, at the prices listed in that document most likely.
You may also want to make entirely sure that there is no decent wired broadband in the area as this will be the best option.