Remember with DSLR's to look outside of the regular exposure triangle (of ISO <> aperture <> shutter speed).
With DSLR's I think you need to make the 'triangle' a 'square' by adding 'software' as the fourth reference point. The software you use (in-camera, DxO, Lightroom, C1 etc) and how you use it (shooting RAW, JPEG only, NR applied etc) needs to be taken into account too nowadays.
A correctly exposed ISO 100 shot, with everything done right according to the exposure triangle, could still look different (visually, and according to the histogram) depending on which RAW or JPEG image processor was used to form the image for display.
Returning to the original posters question:
'Have other K3 users found an "optimum" ISO?', I find it hard to isolate optimum ISO outside of the other features of the exposure 'square'. A dark and blurry ISO 100 image shot as a low-res '1 star' JPEG on the K-3, for example, doesn't tell me that ISO 100 on the K-3 is better than ISO 800.
As a final note: why has no one mentioned DxOMark yet
The DxOMark
K-3 measurement charts are required study for examining the optimal performance of the camera sensor.