the desired output is combining 3 vids, and using scale or pad (do some processing on the input).
ffmpeg -i in.mp4 -i in2.mkv -i in3.mkv \
-filter_complex "[0:v][v0];[1:v]scale=-1:1440[v1];[2:v][v2],[v0][v1][v2]hstack=inputs=3"
-map "[v]" -map 1:a output.mp4
give:
[AVFilterGraph @ 0xec0a80] No such filter: '' Error initializing complex filters. Invalid argument
[0:v][v0];
and[2:v][v2],
meant to do? – Gyan Nov 29 '20 at 5:28