My R3 showed up last Wednesday and I have been comparing the two. Last night I was comparing the spot AF to see if there is a difference between the two. The R5's box seems to be just a very tiny bit bigger, but I notice when the R3 is in the very center a dot appears in the box. I am not sure if that means anything or not. I went around picking tiny objects that might have depth around it and pick out an object in the center to see if it would hold the fine point. For instance one of the targets was this toy https://battattoys.com/product/woode...-farm-animals/ . What I found is that the R3 does have a very slight edge on the R5 but in real world use it may not be very noticeable.
One thing I did is I turned off every function that might track an object. This included servo, continual AF and anything else related to tracking. Even continual AF would jump from the chosen spot.
There is another difference between the two that may matter. The R3 can use eye detect in every AF setting. The R5 only uses eye detect in Face + Eye Detect AF, the R3 doesn't even have that setting. When the eye detect setting is on the camera looks for an eye, when it doesn't find the eye it search's for a face. I saw this quit bit when shooting my black lab pups this weekend, black eyes on a black face and body are tough. Shooting the grand kids it would lock on their eye and if they blinked or turned a way for an instance the AF would hunt.
I think some of the issues I have had is just an understanding of how the new tech works and how to set up the settings properly for what I want to do. This will work out with time. As for the comparison between the 1Dx III and the R5 possibly the R3 and their respective AF I think the names say it all. 1, 3 and 5. Canon has always placed their bodies appropriately and the "1" is still top.