I have the 18-35, and have used it on my K3III (but only about 250 shots total). It has maybe a bit worse keeper rate than my 16-85, but not unreasonably so. On my KS2, that lens worked well in single point focus mode, but not so well in multi-point mode. The K3III seems much more adept at managing the lens. Setting Autofocus to Auto multi-point mode and taking pictures of the grandkids worked out very well. Again, not a perfect keeper rate, but not bad.
I looked through a sequence of 11 shots at f1.8, and only 2 of those weren't focused on the eye. There were 2 more where it was just off the eye, but good enough at normal viewing magnification. 7 were basically perfectly focused on eyes in the shots. These were in good light. Indoors, my keeper rate was worse, maybe 20% were just terrible, and maybe another 20% where it focused on some random thing like a coffee cup. Again, not amazing, but workable.
At f2.8, my keeper rate seems to improve dramatically, with maybe 5% of shots just rubbish, and the rest better than adequate. By f5.6, it seems as good as any lens.
Here's a shot at f2.8 to whet your appetite.