Both Canon R3 and Canon R8 have Full frame sized 24.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Below you can see the R3 and R8 sensor size comparison.
Sensor Size and Resolution Comparison image of Canon R3 and Canon R8 Cameras
Canon R3 and Canon R8 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.
DxOMark Sensor Scores
Both Canon R3 and Canon R8 sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, R3 has scored 96, 3 points higher than R8.
Model
Overall
Color Depth
Dynamic Range
Low-light ISO
Canon R3
96
25.0 bits
14.7 Evs
4086 ISO
Canon R8
93
24.5 bits
14.5 Evs
3295 ISO
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Canon R3
or the
Canon R8 ,
including hyperfocal distance.