Hand 1:

His sizing tells me that he doesn't want us to fold. He's probably doing it with a weaker range, but I wouldn't assume he's bluffing either. Also we can flat profitably anyway, so we might as well do that and save our 4b'ing light bullets for a different spot.

Hand 2:

Flatting is fine. Villain doesn't steal a ton, but we're still doing well enough against the top 25% of hands to flat in the BB. As played, I'd assume that villain is cbetting this flop a lot, and thus would check to him. This is literally the second best bluff we have in our range, and we should have a fair amount of value c/r's here, so I would definitely be checking with the intention of raising.

Hand 3:

I don't think this is great incorporation of the lesson in OP (hey look at me, keeping it on topic). I think villain is going to put us on a weak range, and will be perfectly willing to call with any piece of it, and I think his range is mostly draws and pairs on this flop. We might get like K5s to fold or some shit, but other than that . . . . For this reason, this is a decent board to barrel if we had any outs or any backdoor anything.