MP4Box -cat 1.mp4 -cat 2.mp4 -cat 3.mp4 output.mp4
Appending file 1.mp4
No suitable destination track found - creating new one (type vide)
No suitable destination track found - creating new one (type soun)
Appending file 2.mp4                             
Appending file 3.mp4                             
Saving to output.mp4: 0.500 secs Interleaving