Correct. It saves the tracks as they are on your screen.
If the problem is that your tracks are mono and you want to mix them to two stereo channels, change the pan slider L…R on one or more of the tracks, or use the Track Drop-Down Menu to change any of the tracks to to left or right channel.
Alternatively, Edit > Preferences: choose Quality Preferences , choose “Use custom mix” and click OK. Now you can mix any of the tracks to one or both of the stereo channels.
If the problem is that some of the tracks are not in the mix, unmute those tracks.
See Mixing Audio Tracks for more help.
If the problem is that you need more than two channel MP3, unfortunately standard MP3 is two channel only, which is all that Audacity supports as shipped. You can try using the Fraunhofer MP3 surround encoder as described here: export multi-channel MP3 file - #4 by Gale_Andrews . You can also use this encoder in Audacity’s command-line exporter. This encoder is said to expire at the end of this year.
Or export as multi-channel WAV (set “Use custom mix” as above) then convert to MP3 surround in Winamp .
Gale