A 3X is a 3rd flight added onto the end of the flights you originally wanted to take, so if you want to fly from city A to City B and back you would buy a ticket with three flights
A to B
B to A
and then add a 3rd flight(3X) onto the same ticket ie:
C to D
This can cause the YQ to disappear
YQ is the coding for the fuel surcharge that is part of the ticket price for a lot of carriers, right now UA and CO have a $410 YQ from the US to BA, by adding the right 3X to a ticket you can make the YQ disappear, the trick is finding the 3X's that work for a particular airline as they can be airline specific and also region specific, one that works for the US to BA may not work from the US to EU. also you want to have a 3X that has as little as cost and taxes as possible, because the cost of the 3X is added to the ticket, Right now I have a 3X that works from the US to BA that costs $30 total and removes 100% of the YQ.
Just wanted to give a shout out to Greg and say thanks for having posted this. I had heard about Matrix but wasn't really sure how to use it to my advantage. http://matrix.itasoftware.com/ After you search, make sure you actually click on different boxes when the prices come up. Cheaper prices showed up when I did that. When I compared it to Kayak a few months ago I hadn't actually gone ahead and clicked on any of the airfare prices because it looked as though the prices were the same as Kayak. Anyway, my parents are flying from Dallas in early April. The cheapest fare I found with a less-than-desirable itinerary was $1142. When I searched in Matrix and then clicked around on a few different dates, I got $1020 plus a non-stop flight on the way back instead of layovers both ways. No, you can't book on Matrix. But now that you have the exact itinerary you're interested in, you go to Orbitz or Cheap Tickets (try both - this time CheapTickets was just a tad cheaper) and choose "MultiCity" and plug in the cities and dates. So, for example, instead of saying DFW to EZE, you choose DFW to Santiago, then Santiago to EZE, then EZE to DFW. And it saved my parents a few hundred dollars. Hope this helps!
By the way, that $250 ticket on Mexicana Greg mentioned was no joke! 11 family members and friends got a hold of it back in July 2009 to come to our March 2010 wedding. That was amazing - my aunt had set up an alert on AirfareWatchdog for that one.