ffmpeg -i 1.mp4 -i 2.mp4 -i 3.mp4 -i bg.mp3
-filter_complex
"[0]trim=0:20,scale=1920:-1,crop=1920:1080:0:0[a];
 [1]trim=0:20,scale=1920:-1,crop=1920:1080:0:0[b];
 [2]trim=0:20,scale=1920:-1,crop=1920:1080:0:0[c];
 [a][b][c]concat=3[v]"
-map "[v]" -map 3:a
-shortest out.mp4