ffmpeg
-i discard/video_with_audio.ts
-i discard/slides/audioOnly1.flv
-i discard/slides/audioOnly2.flv
-i discard/slides/audioOnly3.flv
-y
-strict experimental
-filter_complex
[1]adelay=0.1[a];
[2]adelay=1745300[b];
[3]adelay=1758700[c];
[0][a][b][c]amix=4[out] // I think this should be "[a][b][c]amix=3[out]"?

-map 0:v -map "[out]"

-profile:v main
-level 3.1
-movflags
+faststart
-c:a aac
-b:a 64K
-f mp4
final_output.mp4