Ricoh GR III has a 24.0MP APS-C
(23.5 x 15.6 mm ) sized CMOS sensor . On the other hand, Leica Q3 has a 60.0MP Full frame
(36 x 24 mm ) sized BSI-CMOS sensor and features Maestro IV processor.
Leica Q3's sensor provides 36MP more than Ricoh GR III'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.
Another similarity between these two cameras is that both Leica Q3 and Ricoh GR III sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the GR III and Q3 sensor size comparison.
As seen above, Leica Q3 has a 2.4x Larger sensor area than Ricoh GR III. 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.