This rumor still makes no sense to me. So... 2-3 years later they upgrade the sensor, and call it a Mark II. Why? Also, for those interested in getter the stacked R3 sensor, expect the price to jump by $500. Yeah, that makes it a $3,000 camera. That precedent is validated by a number of other cameras that have upgraded to stacked sensors. Not to mention, this would really take a bite out of R3 sales. IMO, the biggest reason to buy an R3 over the current R6 is the stacked sensor. Beside the sensor, performance wise, there isn't a lot in it, so spending an additionally $3,000 for the R3 would get extremely difficult. Don't get me wrong, I'd buy it if it were available but it makes no sense for Canon to do it (at least at this time).
Something really wrong with the CR3 rating on this one to me. If I were to guess, I think Canon is trying to weed out leaks in its system. Canon is very happy to leak stuff on their own when they are ready.