ffmpeg -i first.mp4 -i second.mp4 -i third.mp4 -filter_complex "
  [0:v] scale=640:360 [vscale0]; [vscale0] setsar=1:1 [vsar0]; \
  [1:v] scale=640:360 [vscale1]; [vscale1] setsar=1:1 [vsar1]; \
  [2:v] scale=640:360 [vscale2]; [vscale2] setsar=1:1 [vsar2]; \
  [vsar0] [0:a] [vsar1] [1:a] [vsar2] [2:a] concat=n=3:v=1:a=1 [v] [a]; [a] dynaudnorm [anorm]" \
  -map "[v]" -map "[anorm]" -c:v libx264 -c:a libfdk_aac output.mp4