ffmpeg \
-i http://127.0.0.1:8090/a.mjpeg -i http://127.0.0.1:8090/b.mjpeg -i http://127.0.0.1:8090/c.mjpeg \
-filter_complex "
nullsrc=size=1600x1200 [base];
[0:v] setpts=PTS-STARTPTS, scale=800x600 [upperleft];
[1:v] setpts=PTS-STARTPTS, scale=800x600 [upperright];
[2:v] setpts=PTS-STARTPTS, scale=800x600 [lowerleft];
[base][upperleft] overlay=shortest=1 [tmp1];
[tmp1][upperright] overlay=shortest=1:x=800 [tmp2];
[tmp2][lowerleft] overlay=shortest=1:y=600
" ./test.mp4