Lets look at the rumored specs...
Sensor:
* 24MP (same as R3, R6, R6 Mark II), R3 sensor is 0.1mm higher and wider than R6 & R6-II, but no confirmation on that until details from Canon.
* What that'll mean is that there's likely no observable difference
Shutter:
* Mechanical/Electronic - Same as R6 Mark II
Processor:
* Improved DIGIC X - no details so ... WTF?
Cards:
* CFE (new to R6)
* SD-UHS II (is on R6-II)
Video:
* 4K 120P (up from 60P)
* 1080p 240P (up from 180p)
EVF:
* From R5 (old tech)
IBIS:
* improved without specifics
AF sensitivity
* improved without specifics
Weight
* R6-III weighs more than R6-II
I'm not saying that Canon's doomed, but FFS, the list of what's new/better in the R6III vs the R6II is pretty insignificant. All of the reviewers will rave about it because otherwise they won't get new free stuff from Canon, but man oh man, what a boring camera update. Is this the camera update you push out when you're not really having a camera update? Or is it further confirmation that photography is now playing second fiddle to video at Canon?
To compare sensor between R6II and R3:
From ISO 200-1000, R6II is better than R3, otherwise R3 is better than R6II by at most 0.4 stops (low ISO) and 0.2 (high ISO.)