As in everything having to do with poker, it depends on this, that and the other thing. Then there is some other stuff that you need to factor in. That being said, I can't say that I have a specific or fixed 3-bet range.
As a couple of people have already said, it's starts with observation, what I often refer to as patterns and deviations. My 3-betting range is going to be dependent on these observations. While I'm comfortable raising with any hand in my range, what I raise with is not always the same hand that I would call a raise with. It's going to change depending on the situation. Likewise, my 3-bet range is not always going to be the same. In some cases I might widen it against an EP min raise, not so much to play back at the min raiser as to discourage anyone else from getting in the hand in order to see a flop heads up, because a lot of players aren't deterred by a min raise. I might widen it even more against an agro player who shows a tendency to open light. Where I consider myself to be a tight player, but am willing to raise with any part of my range, some players will call with a lot of hands that I wouldn't even play, but they don't seem to ever raise. So when I see a raise from these players, I might call their raise, but would only 3-bet them with aces or kings.
To the reason why you might not be able to find anywhere where anyone says this is my 3-bet range, other than because it varies depending on the situation, is probably because no one wants to give that much specific information about how they play. I do it all the time. When describing my range I might say that it's tighter than some, but not as tight as others. I will readily tell you that I don't mind raising with any hand in my range. But I will never say exactly where that range begins. I will try to answer questions as best I can without defining my exact range. I would imagine that when people are giving advice on 3-betting, they are trying to do the same thing.