Well, you mentioned "pro build" twice, just to make the list of which the R3 superior is look longer.

))
Anyway, for me who shoots mostly wildlife, it would not be reasonable to buy even a used R3 anymore, which has only the benefit of stacked sensor and pro build over the R6 III. I am not sure if there is a major difference in buffer between these two, 150 RAW pics of the R6 III is plenty even at 40 fps. I rarely photograph birds, so rolling shutter is less of an issue for me. More video options, precapture/prerecording, 40fps, 32 MP, lower price etc outweights not having stacked sensor or pro build, which I also like.
Maybe a sports photographer, who can not afford the R1 would gravitate towards the R3, but still precapture, 40 fps, the price point of the R6 III is very tempting.
I also noticed the increasing number of used R3s for sale just prior to the announcement of R6 III...maybe just a cooincidence.