Its called focus peaking. You have to be in live view, then you choose it in the Menu. I think you can set it up with the Green button, but I don't have a K-3 to be sure. You can look in the manual for more info
Here is the introductory video when FP was first released:
One more thing, when using FP, it is usually a good idea to couple it with digital zoom. And FP is not perfect for all lenses, especially for lenses that are slow (narrow aperture), have really poor contrast (older lenses, or those super-cheap telescopes) or is really wide angle (for example, Samyang 14mm has DoF so wide, FP is almost useless for far subjects)