I would call, I hardly see someone checking a set 3 times here. 10 hands says nothing as sample size.

People rarely x/r bluff rivers, especially on a card thats better for your range and almost no straights/flushes on board. When you xb flop you look like K,A high, so he can slowplay all of his 2p+ hands and after you check turn checking >>> betting river on a K which youre gonna rep sometimes and value bet always, so he achieves nothing by leading.

Never see this line before. I also don't think he has a set here. He probably has 98s. He would like to see a turn bet from u, but u miss it. On the river, the K is a good card to either bluff or vbet if u hit it.

Because of the fact I really don't see that many hands that makes sense here, I tend to call. But I really never see this line and I don't think that a regular bluffs like this.

I would bet/fold here. I don't see to many bluffs + he would probably have bet them himself instead of trying to x/r you.

I folded here too.

I would also BF - this is almost never a bluff - as knoxox said, most bluffs just bet here, as they might not expect you to fold Kx vs XR.

fwiw I'd have fairly ridiculous xR frequency as a villain in this spot, I think that most people do fold their faceup Kx hands to the xr