no such filter, when using filter complex with pre-scale
[0:v][v0] and [2:v][v2] don't contain any filter declaration, and thus ffmpeg complains. Insert whichever filter is desired in between the input and output pads.
I suppose you meant to scale the heights of the 3 inputs to match. In which case,
ffmpeg -i in.mp4 -i in2.mkv -i in3.mkv \ -filter_complex &quot;[0:v]scale=-1:1440[v0];[1:v]scale=-1:1440[v1];[2:v]scale=-1:1440[v2];[v0][v1][v2]hstack=inputs=3&quot; -map &quot;[v]&quot; -map 1:a output.mp4
on input processing with -filter_complex, you use this pattern:
[input_number:v]operation[tmp_output_name]