Canon M3 has a 24.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features DIGIC 6 processor. On the other hand, Canon G3 X has a 20.0MP 1" (13.2 x 8.8 mm ) sized BSI-CMOS sensor and features DIGIC 6 processor.
Below you can see the M3 and G3 X sensor size comparison.
As seen above, Canon M3 has a 2.9x Larger sensor area than Canon G3 X. 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.