Pentax K-3 III Mono has a
26.0MP APS-C (23 x 15.5 mm ) sized BSI-CMOS sensor and features PRIME V processor. On the other hand, Nikon D3500 has a
24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Expeed 4 processor.
Another similarity between these two cameras is that both Nikon D3500 and Pentax K-3 III Mono 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 K-3 III Monochrome and D3500 sensor size comparison.
Sensor Size and Resolution Comparison image of Pentax K-3 III Mono and Nikon D3500 Cameras
Pentax K-3 III Mono and Nikon D3500 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.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Pentax K-3 III Mono
or the
Nikon D3500 ,
including hyperfocal distance.