Yeah I wouldn’t recommend people drink 3 of these controversial energy drinks per day, both for health and financial reasons.

From your link:

Up to 400 milligrams (mg) of caffeine a day appears to be safe for most healthy adults. That’s roughly the amount of caffeine in four cups of brewed coffee, 10 cans of cola or two “energy shot” drinks.

1/3rd of that consumed by a teenager really doesn’t seem that scary, and the page you linked agrees.