Hidden costs on standard bank credit card

Good morning Standard Bank consultant are selling products that they dont have any information about it. On 2018 March 01 I took a credit card with standard bank at phumulani tembisa and the contract that I signed it says I will pay an interest of 3%, but after my first payment on April 2018 half of the amount that I payed was deducted as cash finence charge then I went to Pretoria Central to ask about it and the consultant there did not know anything about it, same thing happened again in May I went to tembisa they also did know anything about it. This year on September I went to tzaneen to ask about cash finence charge and the consultant did not know anythin about it so she put me through the credit card line and everything that was explained to me did not make sense and they sent me a cash finence charge statement. I dnt even understand the numbers on the statement what I saw is 17,50% that is calculated daily I dnt know why. My worry is that how and when will I finish to pay this credit card if every month after my payment they take half of the paid amount as cash finence charge As its being happening for the past two years