SELECT sales.sal_amt, SUM(refund.ref_amt), customer.cust_num, customer.cust_name ;
FROM sales, customer, refund ;
INTO CURSOR rpt013a ;
WHERE customer.cust_num = sales.cust_num
AND refund.cust_num = customer.cust_num

Play around with different syntax from the command line until you get the result set you want.