I have norticed vending machines in the international arrivals of Terminal 5 that sell sim cards plus I have also seen them in Terminal 1 just after arrivals in the meet and greet area.

I would imagine that you will be able to find similar in Terminal 3. If you cant find a vodafone store, try ebay before you go or carphonewarehouse once you arrive.

Alternatively PM me and I will help you get one and send it on to you.