Originally Posted by
janetdoe
I would start here:
http://lmgtfy.com/?q=list+of+airports 
In particular the Orbitz list may be helpful. I pasted it into Excel and it looks like it narrows it down to ~3700.
The problem is that a lot of the 3Xs are between minor airports.
If you need, I have an excel spreadsheet with IATA codes and locations of over 4,200 airports including all the key 3x routes. It also includes another 1,100 airports with only ICAO code, and it also has IATA codes for key train stations (e.g. XMU, XRF) and metropolitan areas (e.g. CHI, TYO).