Both Sony A3000 and Olympus E-M1 III have 20.0 MP resolution sensors but Sony A3000's sensor is APS-C (23.5 x 15.6 mm ) and Olympus E-M1 III's sensor is Four Thirds (17.4 x 13 mm ).
Since Sony A3000's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Olympus E-M1 III.
One other difference between these two cameras that is worth mentioning is that Olympus E-M1 III's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the A3000 and E-M1 III sensor size comparison.
As seen above, Sony A3000 has a 1.6x Larger sensor area than Olympus E-M1 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.