My 2022 3R had a whopping 3 miles on the clock when I purchased the bike new in March 2022. Definitely not a DEMO bike, and likely ridden only by the shop service staff during PDI setup. I would expect them to do that (for safety purposes if for no other reason). Almost 2-yers later and no issues to report.
I understand your concern here, but I personally would not be too concerned. Your bike will come with a full factory warranty, so that should provide you with some peace-of-mind. I would be more concerned about buying a designated DEMO bike from a dealer, but even those are sold as new with full factory warranty.
One of the two Triumph dealers in my area would not even offer a test ride back in 2021/2022, stating that Rocket parts took too long to source if the bike was involved in a test-ride collision. My preferred local dealer allowed me to test ride a 3R prior to purchase (not the same bike I ended up buying there) and that test ride sold me on eventually purchasing my 3R.