user32979: 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


user1871: [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 "[0:v]scale=-1:1440[v0];[1:v]scale=-1:1440[v1];[2:v]scale=-1:1440[v2];[v0][v1][v2]hstack=inputs=3" -map "[v]" -map 1:a output.mp4
on input processing with -filter_complex, you use this pattern:
[input_number:v]operation[tmp_output_name]

32979: <p>the desired output is combining 3 vids, and using scale or pad (do some processing on the input).</p>
<pre><code>ffmpeg -i in.mp4 -i in2.mkv -i in3.mkv \
-filter_complex &quot;[0:v][v0];[1:v]scale=-1:1440[v1];[2:v][v2],[v0][v1][v2]hstack=inputs=3&quot;
 -map &quot;[v]&quot; -map 1:a output.mp4
</code></pre>
<p>give:</p>
<blockquote>
<p>[AVFilterGraph @ 0xec0a80] No such filter: ''
Error initializing complex filters.
Invalid argument</p>
</blockquote>
 1871: <p><code>[0:v][v0]</code> and <code>[2:v][v2]</code> don't contain any filter declaration, and thus ffmpeg complains. Insert whichever filter is desired in between the input and output pads.</p>
<p>I suppose you meant to scale the heights of the 3 inputs to match. In which case,</p>
<p><code>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</code></p>
<p>on input processing with -filter_complex, you use this pattern:</p>
<p><code>[input_number:v]operation[tmp_output_name]</code></p>
