There are at least three different methods of earning commissions under an agency for the sale of real estate (1) by effecting a binding contract of sale, under authority given to the agent to make a contract for the principal; (2) by producing a purchaser to whom a sale is in fact made, and (3) by producing a purchaser ready, willing and able to buy on the terms specified in the agency agreement. McDermott v. Ma-honey, 139 Iowa, 292, 115 N. W. 32.