Canon R3 rumored with a 45MP sensor

Highly unlikely

In the rumor pile, there was an article from TechRadar that is catching attention.

I'm not exactly sure where the rumors are that the Canon R3 (flagship R) was going to sport around a 45mp sensor, as they mention this is what the rumors have suggested.  Professionals that have used the 1 series camera bodies have always pushed back against high MP cameras, and especially with CR3 not supporting sRAW (as of yet), this would be even more of an issue with professionals wanting smaller files for quicker transfers and processing.

Even the 30MP rumor, which is the latest we've heard on resolution means that Canon is playing really tight on the video capabilities and understating them.  It could be that Canon feels this is a sports camera first and formost and really doesn't want the focus to be the video, but it seems odd that it mentions RAW video, yet excludes it shooting in 6k+ (for a 30MP sensor) or 8K (for a 45MP sensor).

However, this all may be true.

Canon develops its own sensors.  The design of a sensor from 24, 30 or even 45MP isn't different in terms of cost, and it's probably low level configuration in the camera itself for the different sensor resolutions. Canon may not have decided and may be waiting on professional feedback to decide on the resolution.  That's one advantage of doing your own sensors is that you can create as many different resolutions as you want to try out in the field. 

With the Olympic games starting shortly, Canon will have an excellent opportunity to tune and tweak this camera body specifically for the needs of its users.  Something neither Sony nor Nikon can easily accomplish (Yes, I mention Sony because in actuality, it's two entirely different business divisions, and Sony Electronics makes sensors to sell, not primarily for Sony cameras). Canon could simply know that it will shoot at least 4K oversampled, but not have solidified what the final configuration will be.

Time will tell!