Sigma Quattro H has a 45.0MP APS-H (26.6 x 17.9 mm ) sized CMOS (Foveon X3) sensor and features Dual TRUE III processor. On the other hand, Canon R3 has a 24.0MP Full frame (36 x 24 mm ) sized Stacked CMOS sensor and features Digic X processor.
Sigma Quattro H's sensor provides 21MP 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 Quattro H and R3 sensor size comparison.
As seen above, Canon R3 has a 1.8x Larger sensor area than Sigma Quattro H. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.