Is 140 pounds overweight for a 14-year-old and if so how do you lose 15 pounds in 3 weeks?

Whether it is overweight is a factor of height as well as age. 15 pounds in 3 weeks is a lot of weight to drop. That's 5 pounds a week, almost a pound a day. There isn't a healthy way to do that, after all it wasn't gained in three weeks. Sensible diet and exercise is always the best way to lose weight. Even 5 pounds in three weeks will make a big difference in how someone looks and feels.