It was a few years ago for me, but this is what I remember thinking back.
800 ISO was the highest "safe" range on the K-5. I dialled that back to 640 on the K-3.
If you shoot a lot of images like sunsets with wide dynamic range, the K-3 was a step back from the K-5.
The 8 FPS and 23 shot buffer of a K-3 is to die for compared to any other Pentax body.
At up to 400 ISO the K-3 images are very cropable. More crop room than a K-5, easily.
At over 400 ISO, cropped to the the same MP, the images are the same as a K-5 for noise.
The K-3 is a great companion camera for the K-1 should you ever get one. They both have strengths the other can't match.
My wife Tess shoots mostly landscape and macro, and still uses her K-5. You'll find the K-3 overkill for many day to day shooting type activities, but last time I picked up a K-5, it seemed so unresponsive and clunky, I could barely bear to use it. I was planning to keep it as a back up, but buying a K-1, the K-1 in crop mode is my back up and is better as a K-5 than a K-5 is. I sold the K-5.
It was definitely time to move on. Now if I could just convince my wife it's time for her to move on.