I just bought an used K3 today.
Compared to my old K50:
*faster screw-focusing motor
*more resolution while zoomed in live view (100%) This is a huge step up for me as I shoot a lot with fast manual lenses.
*more accurate focus
*better lowlight focus
*faster shooting and 4x bigger buffer
*QUIET shutter - K50 sounds like a tank.
So far so good.
Things I have to get used to:
*ISO has dedicated button on top and AF select has dedicated button on the side. Both work by pressing them down and then using the right wheel to change settings. I think the 3 AFoptions on the K50 switch worked better.
*some buttons are in different places.
*can't set AUTO-ISO in M-mode
That's about it for now.