Re: 3D Pop-Left (or Right)
The third top-view image is indeed not absolutely needed.
For example, I (of course) did not have one for this phantogram of Ingenuity on Mars: https://stereopix.net/photo:koUNd1puWc/
However, it simplifies the process a lot, because you do not need to worry about the focal length to get the right correction/aspect ratio.
If you manage to match the reference object in the top-view, the correction is correct.