Bad customer service and false information

Good Day Went to click tramsherd on the 20/02/2019 at 09:05 inside store there is an ad that that says buy any Cadbury 3 and the cheapest one is free, bought 3 and paid for 3 instead of paying for two and the cashier refused to swip my card even though she saw me holding it and told me it cannot be done since she has already scanned... I feel ****med with false adverti*****t, I don't understand why I had to pay more than what was said