Both Nikon D750 and Pentax K-3 have 24.0 MP resolution sensors but Nikon D750's sensor is Full frame (35.9 x 24 mm ) and Pentax K-3's sensor is APS-C (23.5 x 15.6 mm ).
Since Nikon D750'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 difference between these two cameras is that Pentax K-3's sensor lacks 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 D750 and K-3 sensor size comparison.
As seen above, Nikon D750 has a 2.4x Larger sensor area than Pentax K-3. 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.