Canon R3 has a 24.0MP Full frame (36 x 24 mm ) sized Stacked CMOS sensor and features Digic X processor. On the other hand, Canon R has a 30.0MP Full frame (36 x 24 mm ) sized CMOS sensor .
Canon R's sensor provides 6MP more than Canon R3's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the R3 and EOS R sensor size comparison.
Canon R3 and Canon R have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Canon R3 has 25% larger pixel area compared to Canon R, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.