Why are these three items in the debit side of receivables control account?

1. The basic entry everything starts with is credit sale – sale which will be paid later: invoice would be posted Cr sale(net), Cr VAT(if applicable) Dr Debtors(or receiveables control a/c).
Then when the cheque from client is received and you record it : Cr Debtors Dr Bank
But cheque has been bounced/returned it means you need deduct bank and post back this debtor previously cleared- in other words reverse previous entry Dr Debtors Cr Bank

2. interest on overdue accounts – you have had a debtor and because it has not been paid you have charged interest. In other words the client needs to pay not only the invoice as in point 1 but also your interest. The entry would be Interest income Cr, Debtor Dr

3. refund given to customers – it should not be on debit side of Debtor/Receivables a/c. I would say discount given goes to Sale Dr ( or discounts given) and Debtors Cr. Actually it means it is not debtor anymore but creditor – now you are due money to customer, it is liability, Could be shown as decrease in debtors or increase in creditors.
It could be on debit side if it is discount received from supplier – check your question again.