Yes, normally when a camera company states that a lens has a 3X optical zoom, it means it can be zoomed and stopped anywhere from 1X to the max. of 3X and stop anywhere along that zoom range. So, for example, it could go from 1X to 1.3X to 2.6X to 2.9X to 3X, or what ever the user desires within that range of 1 to 3X.
However, with DJI, this may just be how they are trying to describe the optical range of each of those cameras. One being the 1X the second being a zoomed in optical equivalent of 3X and the third the optical zoomed in equivalent of 7X, with no other optical ranges in between.
It would be nice if they were real optically zooming ranges of the lenses though. Ahh, we can dream.