They usually book best through Orbitz or Cheaptickets(which also owned by Orbitz), Expedia is not always good at pricing out a ticket with a 3X properly, sometimes you can book a ticket with a 3X directly on the Airline website if you are using the same airline for the 3X.
But for working out the best fare I use:
http://matrix.itasoftware.com/search.htm
which is a more versatile airfare search engine, it allows you a lot more options, and will allow you to see the fare construction so you can see what if any "YQ" is built into the fare, and when you add a 3X to it you can tell exactly what it is doing to the "YQ", you can also tell if one of the legs of the trip is in a higher fare class, and have a better idea were to make adjustments to get the best deal.
I will give you some help and direction by PM, but the best thing to do is read that thread and start experimenting on your own, when all the tricks are laid out there for everyone to see the airlines are forced to try and close them, whereas if there is a bit of specialized research and knowledge required for the tricks they last longer.
There was an incident about a year ago were Airfarewatchdog.com published an article laying out exactly how to trick a fare and what 3x to use, a day latter a lot of the fares were fixed at least temporally new trick's and 3X's have been developed since, but that is why that thread is written partly in riddles, to make not easily understandable for the average person who stumbles upon it, but if you are willing to put in the work everything that you need is there.
Greg