Pentax K-3 III has a Pentax KAF2 lens mount and currently there are 163 native lenses available for this mount.
The latest released lenses that are compatible with the K-3 III are the Pentax FA 50mm F1.4 Classic, the Pentax FA 50mm F1.4 and the Pentax 16-50mm F2.8 ED PLM AW.
K-3 III also has a sensor based image stabilization system that is up to 5.5-stops effective which means that all of these lenses will be stabilized when used on the K-3 III.
Since Pentax K-3 III has a APS-C sensor, it has a focal length multiplier of 1.5x so you have to multiply lenses original focal length with this multiplier to find the Full Frame equivalent focal length of the lens when mounted on K-3 III.
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Pentax K-3 III Lenses with Image Stabilization
Pentax K-3 III Lenses with Weather Sealings