ffmpeg \
  -i $DIR/cam1-timelapse-$DATE.mp4 \
  -i $DIR/cam2-timelapse-$DATE.mp4 \
  -i $DIR/cam3-timelapse-$DATE.mp4 \
  -filter_complex '[0:v]pad=iw*2:ih[int];[int][1:v]overlay=W/2:0[vid];[vid]pad=iw*1.5:ih[int];[int][2:v]overlay=W/1.5:0[vid]' \
  -map [vid] \
  -c:v libx264 \
  -crf 23 \
  -preset veryfast \
  $DIR/allcams-$DATE.mp4