The contract that you have been sold is the responsibility of the PC World store that you took it out with! as they are the ones that miss sold you the item in question (Three do not as yet sell a broadband with free laptop and they are only supplying the dongle and broadband) If the salesperson told you that the broadband was unlimited then this was incorrect information as all mbb packages have a USAGE allowance (1gb,3gb,5gb, or 7gb depending on the contract sold) this relates to all traffic sent and received over your system. There are a few ways that you can check what you have used! when you launch the dongle the box that comes up with the connect button in it has two sets of figures at the top (sent data and received data in kb) this will give you an idea of usage but a more reliable way is to go to
www.three.co.uk/my3 click the register button type your mobile number (on the box the sim came in) and choose a password, this will then show you a detailed usage!
The store that supplied you with the dongle should have given you a copy of the contract (this will probable have a 24mth min term) and should have told you that you had a 3 day return period!
I hope this is of some help to you!