Kuki is right. The airlines treat an "open jaw" itinerary in the same way as a round trip, provided that it meets the rules of a legal "open jaw" itinerary. The rules actually are pretty simple.
>> 1. The "open jaw" can occur at only one end. Thus, you must return to the city of original departure.
>> 2. The "open jaw" must be the shortest leg of the triangle.
>> 3. You must meet all applicable requirements (advance purchase, Saturday night stay, stopovers, etc.) for the applicable fares.
The "open jaw" fare is the average of the round trip fares to the city to which you go and the city from which you return.
If you want to make additional stops en route, buy two tickets -- an "open jaw" ticket to and from the point closest to your destination and an additional round trip or circle trip ticket encompassing the stops. (The discount fares are also applicable to a legal "circle trip," which can include as many stops as you wish, but cannot contain an open jaw.)