Here's how to mix three audio files into one:
ffmpeg -i alex.aiff -i fred.aiff -i whisper.aiff -filter_complex "[0:0][1:0] amix=inputs=3:duration=longest" out.mp3
inputs=3 part as we're mixing three files.
And this mixes only two files putting one on the left and one on the right:
ffmpeg -i alex.aiff -i fred.aiff -filter_complex amerge out.mp3