Really there isn't much between the K-3 and K-3ii.
The popup flash on the K-3 can act as a wireless controller for an off-camera flash. I like it for use with this for birds and other wildlife:
https://www.pentaxforums.com/forums/22-pentax-camera-field-accessories/256288...-extender.html But most people probably prefer the extra features of the K-3ii, particularly pixel shift and GPS. Also, unlike the K-3, the K-3ii can use KAF4 lenses - the only one at present is the DA 55-300 PLM, but there will no doubt be more to come.
You should also consider the K-70, which has the same sensor and some extra goodies like a flippy screen. It's lighter in weight and takes the same batteries as the K-50. See the recent in-depth PF review. But it has a less-advanced AF system, and lacks other features found in the flagship model, including greater durability, better dust control system, top LCD, dual card slots, extra buttons (e.g. one for ISO), more customization, and many other things.
Whichever model you get, you will find the 24mp sensor challenges you more. It can give breathtaking images, but you need to concentrate on technique: e.g. every slight misfocus shows up. The 24mp sensor is great if you crop a lot (e.g. for photographing birds).