Sony a3500 has a
20.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features BIONZ processor. On the other hand, Nikon D3100 has a
14.0MP APS-C (23.1 x 15.4 mm ) sized CMOS sensor and features Expeed 2 processor.
Sony a3500's sensor provides 6MP more than Nikon D3100'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 a3500 and D3100 sensor size comparison.
Sony a3500 and Nikon D3100 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.