ffmpeg – 3 input videos – where vid2 follows vid1 whereas vid3 is full length – into 2 halves side-by-side video

There were two simultaneous shootings of the same event,
and there was a break in the shooting with camera1, so there are 2 videos from camera1, one following another, with a few seconds missing in-between.
Camera2 wasn’t interrupted so video3 is full-length.

The task is to put a side-by-side full-length video.
There is no difference how would missing seconds be displayed on its half – it may be black or it may display the last frame from vid1 keeping it still, or whatever (although approaches to treat it are welcome).

Thanks