I think it all depends on whether the R3 is generally better than the IDXIII.
It would be hard to see it being priced lower than the IDXIII if it is.
It should have more MP and better focusing - two key things.
The IDXIII is probably only going to be better in terms of battery life. It looks physically bigger - so heavier.
I have a 1DXIII and its a good camera but I always thought that Canon poorly chose to put a 20MP sensor in it.
30MP would have been a much better choice and the camera would be all the better for it.
I'll really be interested in the R3 ability to track items and keep focus.
The 1DXIII was an improvement but still struggles with small moving objects like birds. It's very good with bigger objects.
I await to see what an R3 can do, mirrorless promises so much in terms of focusing but little birds are still tricky.
I actually like that fact that my 1DX3 has 20mp sensor. Images are gorgeous and I can shoot lots of them without worrying about storage size. When editing - TIFF from 20Mp raw file is around 100Mb, can't imagine how big they are when editing 50Mp files... like 250-300Mb... It is like 3-4 images per gig to store. No thanks.
I have 1D mark I, which is 4.2mp camera and images are awesome in terms of both colors and details.
Most people did not tried 1DX3 at all, and just jerking off on specs. It worth the price. Love it with my 400mm 2.8 III. Oh and yeah, I may not be in a mainstream, but I enjoy quality OVF more than EVF. I could have larger keep rate with mirrorless, but I would not enjoy the process as much and you have to really enjoy it hen you carrying 10kg setup over few hour long hike to get that one shot.
I still will buy R3 to play with it and compare, don't have any expectations about it tho.