I have owned four 1D's including the 1DX2 but have no desire to add the R3 to my bag as it would not be a step up from the R5 for my own usage.
I have also owned the Mk1&2 500 and 600mm EF versions but never had any inclination to buy the EF800 for many reasons including minimum focus distance and the inability to stick a 2x TC on it as my IDX2 wouldn't AF with one unless in live view. The 600mm could be extended to 1200 the 800mm only to 1120mm in extreme needs.
I have kept the EF 500mm f4 but use it less often than my RF100-500. I have no inclination to buy one of the bigger RF primes. I love the flexibility of the zoom, the ability to use it with ease in a confined space like the car and the fact I can dispense with a tripod for 99% of my still photography. I love the light weight of the R5 and the 45mp ability to crop. I don't like the fact that the range of the 100-500 is drastically reduced if you stick a TC on it and find I'm doing so less and less but would welcome the launch of a remodelled 100-500 which would accommodate a TC right through the the zoom range.
Big lenses are not the answer to everything, not only for those reasons but also for the fact that the longer the reach the more likely that you suffer image distortion as the air warms up.
So that's my perspective, for me it's not a smart move investing in the R3 and RF800mm f5.6. If I did I wouldn't be posting images of it or the boxes it came in, why would you!? ( Answers not required) I detailed the equipment I have previously owned only to demonstrate my knowledge of using that equipment.