Btn is continuing with ~16% of hands, meaning btn will fold 66% of the time. As such in the above example we can 3b nearly any two profitably
In general though in constructing the above example we want to 3b a pretty polarized range. Even though mathematically 3b a hand like J8o there would probably be profitable, vs a 50% opener we likely do better by flatting it. Also, postflop tendencies of villain are pretty important. If villain is super aggro, the ev of flatting will tend to go down just because we miss most of the time. If villain is passive, villain will be easy to play against post and we will likely be able to get to showdowns easier etc. If villain is stationy in 3b pots then the ev of 3betting some strong hands will go up dramatically while weak hands will decrease. These are just some general 3b theory ideas to think about when you consider what to 3b when.
For disclosure, I'm a husng player so you will have to consider how to adapt this both to the tablesize of the game you play and the depth of stacks (even playing turbos, stacks for me are never >75bb), but the theory is the same -- you just have to consider how to put it into practice.