Both Pentax K-3 and Pentax K-70 have 24.0 MP resolution sensors but Pentax K-3's sensor is APS-C (23.5 x 15.6 mm ) and Pentax K-70's sensor is APS-C (23.5 x 15.6 mm ).
Since Pentax K-70'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 Pentax K-3.
Another similarity between these two cameras is that both Pentax K-70 and Pentax K-3 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 and K-70 sensor size comparison.
Pentax K-3 and Pentax K-70 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.