When three minus one does not equal two!

Checkers recently advertised a product "Buy any 3 and get the cheapest 1 FREE". I bought 3, but at the checkout till all 3 were charged for. When I queried this, I was told that I had to buy 4 in order to qualify for getting one free. I left the goods, and have subsequently spoken to a Manager at the store who confirmed that I had to buy 4 in order to qualify for the promotion. Is this false advertising, or is my maths and my understanding of the English language just not up to scratch?