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Flatting pre here is fine, assuming you play better after the flop in the future.
Flatting flop is bad. There are some draws and you want to build a big pot.
You should generally be playing your sets to make big pots. That means betting and raising, usually on early streets because there are more draws that can call and fewer scare cards that are going to make you wonder whether you're still good and make your opponents wonder whether you outdrew them. That's simplistic, but a decent starting point.
Here, you gave four people a free card on the flop, which has some draws.
Then you gave three people a cheap card on the turn, which has more draws.
Then you didn't know where you were on the river, having significantly underplayed your hand and allowed three V's two chances to outdraw you.
You have to at least call on the river. My gut says raising isn't likely to be profitable here, but maybe I'm all mubsy (like the borogroves).
So, as played, I call.
Pre is fine. Post flop, with 50 in the pot and 240 behind, I'm looking to getting it all in on by the river unless something gross happens. Even King Nit isn't really going to slow me down. Reads always have uncertainties and folding bottom set on a dry board is so weird I think it may be some specific weirdness with that hand rather than a general tendency.
So raise flop, lead turn, shove river would have been my line here.