I started using some preflop hand charts a few months ago. A lot of the hole card combinations fall into a category in which they are suggested to be flat called OR 3bet. When is it better to 3bet and when is it better to flat?

The only thing I can see so far is to 3bet value hands (JJ in BB vs HJ RFI for instance) to punish loose PFR's (and to shorten the hand since we are oop in this example?).

In theory, strategies that have the same EV can be used with a mixed frequency.

So in your example 3 betting and flatting should have the same EV.

In reality there is usually one that results in more EV, but that's usually something you decide on a case by case basis.

It depends on the opponents in the hand, stack sizes, history, etc.

One of the better reasons to mix in reality is it makes your hand ranges harder to read and that can result in EV for you.

In case it's not obvious, you should pick whichever option is likely to have the higher EV. That usually depends upon table conditions and the reads you have on the original opener and any players still with live cards.
Let's say you had AJs on the BTN and were facing an open by MP, who is quite aggressive post-flop. You might decide to 3-bet if villain opens a little too wide, but folds to 3-bets "too often", since the bulk of your EV will come from him folding. You don't really want to call if he's going to blow you off your hand post-flop. It's especially important to 3-bet if there is a tough/aggro player in the BB. You don't want to flat if the tough reg will overcall or 3-bet squeeze you out of the pot.
Now consider different player types. e.g. MP is a tighter opener and weak-passive post-flop (so you'll have less chance of making him fold to a 3-bet, but you'll be able to realize your equity cheaply post-flop), and the BB is a very passive fish that will overcall with hands as weak as A3o. In this spot, calling has more merit, because you'd like to play post-flop against weaker players.
It basically comes down to whether you think you'll make more money by making everyone fold pre, or by seeing a flop cheaply vs wider ranges. If you don't think the 3-bet will win immediately very often (e.g. because villain is a station) don't 3-bet light.

Thanks Arty. The AJs on the BTN vs MP scenario I think I'm more likely to face a maniac villain who will call the 3 bet too lite there. Sometimes you gotta be willing (eager?) to stack off lite. That's how it often plays at 50nl Ignition. Why we practice healthy BR management

What about in tournaments? I always wonder what's the ideal balance between being aggressive/3betting and preserving your tournament chips. Like you don't want to risk your life all the time do you? But you also want to be aggressive because isn't that the best way to win? What's the answer to this paradox?

Perhaps it's best to be aggressive because you'll bust more but when you don't bust you'll usually go much deeper?

What is the overall 3bet strategy in tournaments anyway at a high level? To 3bet more often to increase variance? Do we want to increase variance because you win more that way?

opening stages with 100-200bb stacks effective: I just try to choose the line with highest chip ev since the money is so far away.

middlegame: 50-100bb effective: 3 bet less often than above because losing your stack is more likely to happen with less effective stack.

bubble game: <50bb effective: you should have some very solid reads by this time. If they're gonna call you often? You need a big hand to 3 bet. If they're gonna fold often, you can 3 bet much more often.

endgame in the money: you should have great reads on everyone by this time(hint: if you don't have a read? they're probably very tight). With reads you can either call a lot of hands or 3 bet a lot of hands or fold a lot of hands.

Thanks. Although the middle game's a little fuzzy everything seems to make perfect sense. Thanks Bob.