I have a doubt that it could be seen.
These are much smaller than the ISS, but more importantly, they are in medium earth orbit at about 20,000 km (12,000 miles). The ISS is by comparison orbits at 350 km.
Still, I would not rule it out for a ground based instrument, though I think you would need more aperture than 100mm.
Each solar panel on a Block III satellite is about 5m, so if you added the entire width of the panels and bus, you would probably get an angular size of 30m, so at 20,000 km, the angular size would be a little over 5 arc minutes.
The height of the panes is also about 5m, so in the other aspect, the angular size would only be about .52 arc seconds.
The problem though is that these figures are for a "solid" surface, and the configuration is two panels to a side with the bus between.
For this reason, we would probably be better as taking the size of a pair of panels, and these are about 5m by 12m. Now we have about .52 arc seconds by about 104 arc seconds and at this size, I think that it would be beyond detection in a 100mm scope.
There is also this. Not only is the size small, but the first rule of contrast transfer is that a bright line against a dark background will appear wider than it is, but a dark line against a bright background will appear narrower than it is.
If we consider the two panels to be a small "Dash" that is .52 arc seconds wide when stood on end, we can see that diffraction of the aperture is going to make the .52 arc second height (now called the width) diminish to something far less than this.
Now I think it would be possible to image with an amateur size instrument, but I think you would need to be imaging, and I think you would need to be several times the aperture you have.
But I am after all just guessing. I have never tried to see it with a 100mm instrument, or any size instrument to be perfectly fair about it.
Edited by Eddgie, 15 August 2019 - 08:16 AM.