Compile an MP3 file

Well for a start, you don't have a track with an actual MP3 in it, because Audition cannot place those directly into Multitrack view; it only operates with wav files. So what you will have, however Audition opens it, is a decoded wav copy of your file. If you are only getting one track being saved in your export, then the chances are that you have the export properties set incorrectly - or you've done something strange with the session. Your safest bet is to go to Multitrack>Mix down session to a new file, and this will open up in Waveform view, so you can check that it's correct. You can at this point save it as a wav file, with a suitable name. Now if you go to Save As, you can resave it as an MP3, safe in the knowledge that it's okay.