I'd 3bet/fold...because:
1. 3bet calling ranges are usually wide at my stakes (50NL) and 4bet ranges veryvery tight.
2. I'm far from comfortable playing JJ OOP against 2 opponents, I'm quite sure I'm not playing it good enough in that situation to yield more profit than 3bet/folding.

I used to call because I thought I more or less turned my jacks into a bluff, but reason 2 made me stop doing that.


But it's mostly preflop reasoning, and no doubt there's important postflop aspects on the decision that are usually overlooked / too complex for most to figure out.

I am really interested in the 'cbet tightening showdown ranges' part of you Fnord.