these questions are too broad, its hard to answer when you dont supply enough basic information such as positions and what you think "light 3bets" are, what your 3betting ranges are aside from the light 3bets, etc.
I would be looking to exploit people who fold to 3bets more than 60%, by expanding my 3betting range, which is already fairly balanced and does not only include QQ+ and AK type hands in the first place.
Generally I like to have a hand sample of 10 or more instances where villain gets 3bet and folds more than 60%, the more the better.
If someone thinks of themselves as a reg, they will have a hud 90%+ of the time. This is nothing that I can prove, but really theres no reason for a player that wants to improve their game would neglect such a widespread and easily accessed tool that their competition would be expected to have as well. People learning the game intending to be regs may not have a hud yet, but its only a matter of time for most people.
I have no idea how much light 3betting affects winrates, because i dont know what you consider light 3betting. But 3betting in general is extremely profitable