Fujifilm X-T3 has a 26.0MP APS-C (23.5 x 15.6 mm ) sized BSI-CMOS sensor and features X-Processor 4 processor. On the other hand, Pentax K-3 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Prime III processor.
Another similarity between these two cameras is that both Pentax K-3 and Fujifilm X-T3 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 moire occurring in certain scenes.
Below you can see the X-T3 and K-3 sensor size comparison.
Fujifilm X-T3 and Pentax K-3 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Pentax K-3 has 7% larger pixel area (15.24µm2 vs 14.12µm2) compared to Fujifilm X-T3, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.