A through fare involving three airlines is likely going to be the "YY" (or IATA) fare. Hardly anyone actually uses these fares as sum of sector is often cheaper. However these fare do apply on any reasonable direct routing (ie within the MPM) involving any combination of participating airlines and do permit rebooking/rerouting without charge at any time. Some people are willing to pay the gobsmacking price for this flexibility.
Travel agents also use them to show their customers how much they are 'saving' them.
The ITA software I linked above is a great tool for finding flights/connections and fares. While they won't sell you a ticket they will give you the fare calculation code that you can give to your agent. E-bookers was happy to issue me a ticket based on their calculations. It isn't easy to use though. Here is a beginners guide:
http://www.theflightdeal.com/2013/02...-ita-software/