It's important to distinguish rolling shutter in photo mode (assisted by DRAM), and in video mode (direct ADC IO)
In photo mode, R3 has a electronic shutter flash sync speed of 1/180s at 14bit, indicating a sensor readout speed higher than that. (A1 e-flash sync is 1/200s and sensor readout 1/250s at 14bit). This aligns with the official claim that the rolling shutter is about 1/4 of 1DX III (1/60s at 14bit).
In video mode, the full sensor has a readout speed of 1/50s at 12bit, this enables a 16:9 crop of 6K at 60fps.