According to the specs the R3 AF at EV -7.5. Both the 1Dx III and the R5 say AF at EV -6. If we believe in the spec fairy the R3 should do better in low light, sometimes the spec fairy doesn't deliver but maybe this time it does.
I did a small amount of research to see what this really meant and how it was calculated. If I do not have wrong information, my understanding is that the EV low light value is based on shooting wide open with a extremely fast prime, like f/1.2. As the aperture gets narrower the AF EV value goes up (gets worse). If that is true in Johnathan's instance using the 100-500mm the poor low light AF performance would be magnified because of the small aperture.
With the 5Ds R it had a EV -2, so I really didn't think about the AF of the R5 because it performed as well or better than the 5Ds R.