My guess is R3 being the high megapixel pro camera.
The later R1 might be using the rumored 21MP global shutter sensor.
I also assume Canon generally will keep a 4 years release-frequenzy for their high-end pro camera(s). Ignoring the current scheduling chaos from COVID and general abnormalities caused by change of technology from DSLR to mirrorless, I'm guessing one camera released up to FIFA World Cups and the other body up to Summer Olympics. That could mean development-annoncements and release years like:
1DxIII: 2020 (Originally scheduled OL year)
R3: 2021-2022 (WC)
R1: 2023-2024 (OL)
R3 II: 2025-2026 (WC)
or maybe Canon take the chaotic times (including high level of technology changes and progress) as a opportunity to swap, so R3 are will be the "olympic" body and R1 aligned with WC:
R3: 2021 (rescheduled OL)
R1: 2022 (WC)
R3 II: 2023-2024 (OL)
R1 II: 2025-2026 (WC)
Yeah, all just speculations
But a global shutter sensor in R1 would clearly differentiate the R1 and R3 series.