If the £ sign doesn't appear on the 3 key (and, indeed, appears to be missing altogether) you've obviously got a non-UK keyboard. There are several dozen different QWERTY layouts used throughout the world but none of them, that I know of, has an asterisk (*) on the 3 key; the international standard is for a hash (#) to be placed there, so I'll assume that's what you've got .
However if pressing Shift 3 produces the £ sign your computer is configured to
think that you've got a UK keyboard anyway. If so, pressing Shift and the comma key together should give you the @ symbol.
Your alternatives are:
1. Simply learn to live with it ;
2. Use stickers or Tippex to change the key labels ;
3. Get a new (UK) keyboard - possibly best!!! ; or
4. Tell your computer that you've got a US keyboard (since that's the international standard). On the computer I'm using now, that's achieved by clicking on 'EN' at the bottom right of the screen, which gives me the option to change from 'English (United Kingdom)' to 'English (United States)'. If your computer has that option you can use it as well. Alternatively you'll need these instructions:
http://tinyurl.com/pbfprzp
The keys on your keyboard will then match what is typed. (To type a £ symbol, press AltGr and 3 together).