I bought three £10 cards from Asda, they were not working so I went back to asda but they would not give a refund or any sort service. 3 days have past now and I really want what I paid for. How can I get my £30 for the store?

If the store won't properly activate them (I assume that by 'not working' you mean that you are getting a message about them not being activated) then try contacting iTunes Support, and say that you've tried going back to the store (you will probably need to give them images of the fronts and backs of the cards, and their receipt(s)) : http://www.apple.com/uk/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then iTunes Cards And Codes

You could try contacting iTunes Support anyway and see what they say