Actually, you can. I made many maps that had more than 3 soundtracks. Its actually really easy, all you gotta do it just place the music you want to be in your map, in FH>Media>Music.
After you placed them there, go back to the normal FH folder, and open music.cfg. Create a new group, ex: [Waffle]
After u did that, hit enter and type this: media/music/YourMusicFilenameHere!!.mp3 or .wav
And instead of YourMusicFilenameHere!! place the real name of the music's filename
I made my own group for example:
[MyMusic]
media/music/credits.mp3
media/music/sal.mp3
And so on
Save it, then open your FH game, type the group's name in the 'Music File/Group' thing.
Ta-da! Now u will have more than one music playing in your map! C: Oh, and btw, the music wont play in the order you placed it, it will just play a random one from the group.
Hope this helped!