At the high school level -- not talking about super teams here -- 28 to 30% is good. The math:
*A 40% shooter from inside the arc produces 80 points on 100 shots. And if you have more than one 40% shooter on a "normal" high school team you have a good shooting team.
*A 27% shooter from outside the arc produces 81 points on 100 shots.
There are a lot of other factors. For example, the inside-the-arc shooter will get to the line more, but will also turn the ball over more and have more shots blocked.
Here's what I tell my players: If you make one out of four three-pointer attempts, you are as effective as a player who makes 37.5% of her shots from inside the arc -- and 37.5% is good shooting, again for normal teams. (A lot of shooting percentages for good teams are elevated by steals and athletic plays that result in high-percentage layups.)