3 for 2 specials - FALSE ADVERTISING

I went to Clicks N1 City last month and Clicks Richmond Centre yesterday. Both occasions I bought the 3 for 2 special on the Essential vitamins. On both occasions I was charged for all three products. Before I left I checked my till slip and asked for a refund on one of the products. Clicks please check your systems on your specials... this is false advertising and why must I double check every item I buy?