I'm editing a three-camera shoot. An AE made a multicam sequence before I started the project. One scene is a song and there are three takes. All three cameras rolling on each take. Is there a way to stack the three takes (so three multicam clips) to make one, nine-camera multicam sequence?
Was the song being sung to a recording? If so, you can just multicam all nine cameras to the recorded track. Otherwise, you can right click each multicam sequence, Open In Timeline, and copy-paste and manually sync each three-camera group into the primary multicam sequence you want to use.
If the song wasn't being sung to a recording, don't bother--humans introduce too many tempo and pitch variations to live performances for this to be truly a useful way to edit.